Noise in the cab at one rpm only.

I have a 1988 Toyota, carbureted 2WD pickup. The truck has a very quiet exhaust system but at a low rpm just above idle it makes an annoying "vroom" sound in the cab each time it passes through that rpm. If that rpm is held steady it will continue. There is no such sound at any other engine speed. The truck need not be moving. It is so annoying because that rpm occurs so much in city driving that the "vroom" is coming and going constantly. It's hard to guess, but the rpm seems to be around 1000. Maybe a little higher. I've searched the groups for advice and thus have jacked up the engine while watching all the motor mounts, and all the following. The exhaust system is in good shape and properly supported. My efforts to locate the source of this noise have been mostly by doing things that would stop the noise so I could figure out what to do. I have weighted the exhaust pipe at different places to stop any resonating(?) I also put the jack under it for the same test. It had no affect. I hooked a hoist to the front of the engine and pulled it up hard, but it didn't help. I weighted the engine down about a hundred pounds to no avail. I took off the A.C. and smog pump belt. That made no difference. I held the fan still while running the engine. No help. The 22r just seems to love to vibrate at that rpm and it sounds like someone beating a drum in the cab. Something is resonating. Any suggestions? I am so stumped that the only other thing I could think of was to come in the house and join this newsgroup and ask you. It's my first post. Thanks for any advice you have.