Okay. Now this is going to be awkward for me, and tedious for any
reader. I know that. And I really think my "problem" is so unusual
that it may be pointless for anyone to read at all.
Bottom line: If you read it, thank you. If you don't, God knows, I
understand why!  ;-)

 I have this "thing" that I call "thumping".
 I have no idea what it's called (or even if it has a name), so I
can't go look it up somewhere.
 It's difficult to describe, and this is why this is going to be
awkward for me. I've asked my doctor about it, and I'm not honestly
sure if I got the point across to him. (since I'm unsure, then I
probably didn't)
 The weird thing is that I've been thumping for years, but because I
am virtually unable to describe it to anyone, I don't try very often.
And since it hasn't killed me, I assume it is non life-threatening.
 So what's different now that makes me go through the trouble? It's
because I started treatment about a year ago for my depression, and
serendipity struck. One of my drugs made it go away, and I didn't even
know it wasn't there until it came back when I ran out a few months
ago. I did some self-testing (much to my doctor's chagrin), and if I
stop taking "muh drug" I start thumping after about 2 days. When I
resume muh drug, the thumping stops again within a day or so.
 I firmly believe my thumping is neurological in nature, but I'm not
ready to call it some form of epilepsy or what, but here is as good a
place to start as any. My grandest hope is that someone will
immediately recognize my awkward description and say Hey! I do that,
that's just a case of such-or-such. Nuthin' ta wurry 'bout....
 But if this thing is as much a  PITA to any other person as it has
been to me, I would enjoy the priveledge of offering a possible
"cure".
Effexor, an SSRI used for depression, eliminates it 100% for me. I
know drugs are often effectively different for different people (e.g,
I can eat both Sonata and Ambien like they were candy). In my case my
Rx is 300mg per day (Umm, that's for the Effexor, not Sonata or
Ambien<g>) It may be effective at lower doses in me or others, I don't
know, but it works for me at 300.
(officail disclaimer: I'm a highschool dropout who doesn't think his
humerus is very funny at all. Obviously I ain't no doc. Talk to one
before you take my word on anything I ever say. Stupid lawyers. I
gotcher disclaimers hangin. Heeeyaaah!)

 And now for my descriptive routine.....

For those who understand hydraulics:
  Imagine taking a syringe with a piston about the diameter of say a
quarter, and implanting its say, 1/8" diam. or so needle into a vein
or artery such that the syringe becomes an "off-line" resorvoir of
blood or saline or ratpoison or whatever, okay? Ya with me? Okay. Now,
just like you might knock on a door or tabletop, neither like a heman
nor a wimp, knock once on the syringe plunger. There! Did you feel
that quick "thump" throughout your entire body, (but most noticeable
in your head and torso) as though the hydraulic system that we all
know and love as our circulatory system experienced about a 10ms
overload?
 That's kinda like my "thump".

For those who love playing with HV systems on TV's and Lasers:
 This is more detailed, but a LOT closer to what I feel.
 Ok. You're sitting on the floor playing with your gadget. The system
has been off and unplugged for a few minutes now, and you reach over
for that thingamabob you need that just happens to be sitting right
there next to the anode.
GOTCHA!
 In slooooow motion here now:
20KV has just discovered its Great NW Passage to ground via the
pathetically small conductance of your house framing, the floor, the
rug, your butt, your body your arm and the back of your hand that
brushed that damned f'n wire!!!
 As it discharges, the flowing current takes complete control of your
body (as low as it is, it is still thousands of times greater than
anything your body's standard issue bioelectrical system can deal
with) For as long as a couple of milliseconds, your body, including
your brain, shuts down completely except for the physiological
reaction of your musculature along the path of discharge. Your arm
muscles retract violently and your entire arm folds back with
incredible speed and power.
 Okay, back to real time now. The previous description took, as I
said, a couple of milliseconds. You can't even count to 1 that fast!
 That box you live in called a body is one of the most wonderous
machines imaginable. The current has now stopped flowing, and almost
immediately your brain has critical functions back on-line and is now
communicating with the rest of your body. Your heart is hard at work
rushing oxygen back to momentarily starved elements (not the least of
which is the brain itself) and your brain has ordered an increase in
adrenalin to assure functions are trucking at maximum capability until
any danger is past. Your entire forearm aches as though you just did a
couple of hundred curls with a twenty pounder.
 Here's the interesting part: up until this point you were
unconscious of anything that was happening. As your brain takes back
control, it reconstructs everything so that you KNOW just EXACTLY what
happened.

 "Yeah, yeah...blah,blah...so where's yer steenkin 'thump' at in all
this rhetoric?"

 Funny you should ask! My "thump" is almost the exact equivalent of
my brain coming "back on-line" as noted above, EXCEPT: no aching arm,
no adrenalin rush (after all, nothing dangerous preceded this point),
no racing heart. That's why this damned thing is so hard for me to
describe. Nothing at ALL precedes my thump, it's almost as though I
lose consciousness for some fraction of an instant, of which I can't
be aware, because in my goofy theory here, I'm unconscious!
BUT!
I *AM* aware of "coming back on line"! And it happens with a kind of,
hell, I don't know what to call it; a freagin "THUMP"!

Now that I've described a singular "thump" to the best of my ability,
I should also add that by no means are my thumps constantly with me.
Over the years they have become a pretty much everyday occurrence
(until my Effexor epiphany, that is), but they are just not constant.
(BTW, I speak in present tense, but, if you will please forgive a
horrible pun, my Effexor renders the problem total non-tense).
I've seen much use of the word "trigger" here in this group. It's as
good a word as any to use.
If I am sitting down reading, or on the computer, or watching TV or
some such, when my thumps happen, they will be triggered most often by
my changing my attention. For instance, I'm a 2-finger typist. If I
start thumping, it will happen when I look up from the keyboard to see
what I just wrote. And then again when I look back at the keyboard to
type.
Another anomoly is that except for relatively predictable "triggered
response" as I just mentioned, they can also happen spontaneously in
clusters.
For instance, I'll be walking through a department store or some such
and thump..thump......thump....thump..thump....thump
That is to say, they will occur in clusters, usually 3 or 4 but often
more, but I doubt ever more than a dozen or so sequential thumps.
Nor are the sequences evenly spaced. The above dots between thumps are
ersatz timing clicks. The above sequence might occur over about 3 or 4
seconds. And then they go away for 5 minutes, 5 hours, who knows
before it happens again.

Anyway, there it is. I've exhausted just about every angle of
description I can come up with, as well as anyone who has read this
far, I'm sure.  ;-)

I REALLY hope I'm not being overly tiresome to anyone here. I'm
sincere when I say that. I know me, and how excruciatingly detailed I
can get on the simplest of subjects.

But so help me, this thing is real, its bugged me for years, and now
that I know it can be completely controlled, I want to try having a
mutual backscratch by having somebody effectively recognize what I'm
talking about, compare notes, and if this is an identifiable
phenomenon, I can learn more about it and let anyone else who "has"
this know what worked for me.

G'nite.
Cheers and thanks,

    Mike