87 Accord Automatic transmission SHAKES terribly while in drive and stopped -- help!
 

I've worked on this car a lot. 1987 accord, carb, auto trans, with
very low miles--50,000. Yes, 50,000 original miles after 20 years.

The remaining problem now is that the car shakes and rattles while at a
stop. When it's in park or neutral, it's not bad. But when it's in
gear it sounds like a jackhammer, so I generally have to shift out of
gear at every stop light. The shake isn't visible from outside the
car, nor is it apparent while looking at the engine, but inside the car
it sounds like a jackhammer. Loud, as the dash rattles all over. The
dash is not loose, nor has it even been removed as far as I know.
While driving or moving over 5mph, there's no abnormal noise. But when
stopped it's terrible. Same in reverse or other gears. Otherwise it
works great.

It passes smog here in california, which tells me that it's pretty well
tuned. When the idle is over around 1100, the jackhammer is much
quieter, but the car is tugging and that doesn't feel right. When set
at factory recommended idle of 750, it's awful.

My only remaining guess is that it might be the transmission fluid. I
changed it over a year ago and it didn't really make any difference. I
used dexron 3, or mercon or whatever the manual says, but it's not oem.
If anyone is sure that the oem ripoff fluid will fix it, I'm happy to
pay the $30 extra or whatever, but otherwise I don't want to waste any
more time & money. It's not due for a change. It just seems to me
that there's so little wear on the car, that it HAS to be easily
fixable. But for all I know, there's something wrong with the trans.

This car belonged to my grandmother, and I don't remember it being
noisy at all, but as long as I've had it, it's been fairly loud. Are
these cars supposed to be fairly quiet inside the cabin? Is it
possible for a non-standard muffler to cause this stuff? Help please!

Thanks.
Greg