Deathbots

Bear with me here - long post but the results could mean that I'm not paying back an auto loan for the next 2 years on a junk car. I really need some help from the experts.

I looked at this 86 gxl today to buy, and overall everything runs great. The engine is supposedly rebuilt, and it sure runs like it. The trans is supposedly rebuilt, too .. BUT

I get in and shift into first, drive away to take it for a test drive. It starts making this noise, though. Imagine rocks hitting a thin metal? But it's not rocks. It's wierder than that, and honestly I don't remember it all that clearly. It also started out pretty roughly, like it had a bad U-joint or something was my first reaction. It doesn't make the noise unless I hit the gas. If the clutch is engaged and the trans is spinning, it doesn't do it. It did this noise after accelerating through second, too until I got up to speed. After that, everything was fine. Even up to 90 mph, where I finally decided to stop accelerating on the small, near one lane country road. I'm telling you, apart from this noise, it runs beautifully. No Horrendous vibration at 90.

I was scared that it was the trans, though. The thing is, it probably really was rebuilt - the clutch is perfect, and the cars in good shape, so the owner seemed like he took good care of it.

The guy said it was his son's car, and he was just selling it for him, he didn't know all that much about the car. This seems to check out, since it had colorado plates and I'm in illinois. But there was a greenskeeper (he was golfing and this car was in a golf course parking lot) who was a mechanic, and I asked him to take a ride and tell me what he thought. So I said, trying to be all worst case scenario - it sounds like the trans. And he goes - it's definately not the trans, sounds like the U-joint.

Now I've never heard of a U-joint making noises before, and I don't know all the specifics of what happens when a U-joint fails. So can any of you tell me whether or not this guy was lying? Can a U-joint make a noise like that when it's failing? Any noise at all? Overall, the car shifted smoothly, and like I said, I got up to 90 with no problems after like 25. It wasn't gear specific because it did it in 2nd and 1st. But I have to know if it was the trans or not to make my purchasing decision. I hope it's not, because it's a nice car and I really want my Rx-7! I can afford an aftermarket driveshaft, not a rebuilt trans

Thanks for reading this far!

--edit--
realized this should have been in tech support. sorry! it's there now