Mystery noise.

My '68 Chevy is making some funny noises lately. I hear a metallic thumping when accelerating. If I let off the gas, it stops. I thought maybe low oil. Topped it off, but the sound persists. It's not the motor at all. I'm thinking maybe a UV joint is going out, since the thumping only happens when I give the van gas to move. The more gas I give and the more speed I collect, the louder and faster the thumping. Also thought about the tranny, possibly a throw out bearing.

Anyone got any ideas?

6 cyl? If its a tick, mine does the same thing, mechanic says the lifters get varnished and stick on these engines, try a bottle of lifter/valve quieter but have an experienced mechanic listen to it, could be a number of things, a low pitched thump could be a U-joint as you said.

Make sure none of your buddies are messing with you and tied something around your drive shaft .

Heres one I had experienced!!!!!!!!!!!Had about the same type noise once - after much time I pulled the 3rd. member cover on rear end 7 found the shaft that secures the spider gears loose & was hitting the sides of case!! on a gm rear ends there is a "pin or bolt" that secures the shaft in there on some models-look in there! use a magnet to find the bolt & metal in bottom oil area.

It could be the fan hitting the shroud. If you think it is the u-joints, go under and try to wiggle the drive shaft up and down or sideways. Any play will mean the u-joint is bad

Went out and wiggled the drive shaft this morning. Solid as a rock. The fan is good as well. Talked it over with my dad, he thinks it may be a rear rod in the motor. The sound is definitely sounding like the back of the doghouse. I don't know.

Any noise at all in neutral while idling? If it was an automatic I would say check the bolts on the flex plate. Not sure what a loose flywheel sounds like. Gears on the input shaft of the trans can sound like a main bearing knock when they get beat up. Been doing hole shots ?

To ccheck a thor out bearing, there must by a little free play in the clutch pedal, not just stopped but during acceleration feel for the free play. If free play is there, then just take up the free play with your foot when the noise is happening to listen for a big change in the sound. No change probobly means its not the throw out bearing.
A drive shaft with a bad u joint will not always be loose. sometimes the bad u joint cut is packed with rust and siezed bearings.
If you think it is a bad rod in the engin, pull the spark plug wire off the one you suspect and then try to make it make the noise. If you can no longer get the noise that will be a clue the problem is in that cylinder.
another consideration should be a loose harmonic ballancer, loose clutch plate or loose flywheel to crank bolts.
M1D

Now that I think about it, before I added oil, the thump sound did happen, a few light thumps while in neutral, when I gave it gas. After oil, there is only 1 light thump. Actually, the thump is more of like a metallic tap.

If I accelerate easy on the gas, the tump isn't as noticeable.

any thump?Noise=is BAD!! find out what it is !1 or if you cant determine whats causing it?????????????

DRIVE it =HARD!! it will show you whats wrong!! = seriously = once again!! = LISTEN TO = DAD!!

One time I had a noise that everyone thought was a rod bearing. It was a metalic noise, and definatly sounded like a rod nock. It turned out to be a cracked flex plate. Chevy with automatic trans has holes in the flex plate. The flex plate had cracked from hole to hole. There was only one that hadnt cracked that was holding the flex plate together

Is there a flex plate in a manual transmission?

No. You have a flywheel. It could be loose, it happens. Only thing is, to check some of this stuff some disassembly may be required, I would also recommend a mechanics stethoscope. might help isolate the target area better.

Ok, went out and drove the van today for the first time since 8 May 2015. Took it about a mile or two down the road, to Wal-Mart. The speed limit was 40, but I did about 35-ish, or so. Still heard the noise, which the more I listen to it, it actually sounds like a nail or key tapping an empty aluminum can. If I let off the gas completely or back off the gas a little, it gets quieter and stops. And if I rev the motor a little in neutral while I am warming up the motor, I hear a couple taps, and what I think is a slight backfire.

The sound is coming from behind the D & P seats, so I'm thinking what I will do is have my son or my dad sit in the back while I drive a few laps around the block, as it will be easier for them to pin point it, as I have to concentrate on the road. While I do this, I'm going to pull the plug on one individual cylinder at a time, maybe that will pinpoint which cylinder, as M1DADIO suggested.

I kind of want to do the simple stuff first, before I go wrenching on stuff.

Thanks everyone for the suggestions, and I'll keep you guys posted.