New 1st Gen Owner, trans issue/question
 

Excited to be a part of this forum... I've spent hours learning from all the posts, what a great resource! Hopeful I can contribute as well! I just purchased my first, 1st gen. Details are in the signature... but in short, its a 1993 D250 with an NV4500... truck is stock other than a few adjustments by the PO (fuel pin rotated, power screw turned 1 turn, timing bumped 1/8). I just installed pyro, tach, and boost gauges (coming off my 2014 GMC I felt naked with no gauges!). I've been told that all fluids were changed within the last 5k miles, and a new rear main seal was installed at the same time.

I've read 100's of posts trying to figure this out on my own, but at last, I need some guidance. I'm experiencing what I'll describe as a "thumping" sound that you can feel in floor boards, steering wheel and pedals It appears to be consistent with engine speed... the more you rev, the faster it thumps.

The sound/feeling is present when parked in neutral with the clutch engaged... and is quite loud. If I disengage the clutch while parked, the symptom goes away.

While driving, the sound/feeling is present in all gears, but most noticeable in 3rd, 4th and 5th when accelerating. I can hear/feel it at all RPM's while driving, but it's most pronounced starting at about 1100 RPM up to 15-1600.

I cannot hear/feel it while looking under the hood, or while laying under the truck. I don't know how else to describe it other than the "thump thump thump" sound... almost like a compressor but deeper.

Would appreciate any insight/ideas/thoughts! Thanks in advance!

(here's a pic just for fun)
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https://cimg9.ibsrv.net/gimg/www.die...666f729f6a.jpg