whump - whump

I am at my wits end. I have a 94 MX5 that has a very annoying problem. It has a noise that I can best describe as a whump-whump noise. It speeds up and slows down as you speed up and slow down. It is not engine noise as I can be driving at speed, push in the clutch and the noise does not stop. I changed the tires, and rims to no avail, the brake rotors seem ok. There have been periods of time when the car sits for prolonged periods(months) without driving. I don't think it is a bearing because it has done it for several years now, and has been on some long trips at interstate speed. PLEASE HELP, it takes a lot of joy out of my toy.

i dont know if this will help but when I got my miata the rear wheel bearings were shot and made a horrible noise when going down the road I dont know how many miles you have on yours but the bearings are like 40$ and yer gonna need a huge 3/4 or 1 inch socket set to get that massive nutoff the spindle.
It also could be a universial joint going bad I know there hard to jack up and get under but its worth a try to get under it and check for side to side play in the drive shaft it could also be the rear bearing in the transmission just haft to starting checking and eliminating.