I have a metal-on-metal rattle coming from the engine compartment. The engine is a 350HO with a ST-300 automatic. The trans has just been rebuilt so that shouldn't be the source. The noise is more noticeable when the engine is hot. It doesn't make the noise when idling or revving while parked. It occurs when moving and applying a slight amount of throttle after coasting. It also occurs at constant speed, and to a lesser degree under breaking. I'm thinking that it might be a timing chain or possibly the starter but wanted a second opinion before tearing into it. Thanks for your help.

If the noise appeared the same time as the repair, there's a very good chance that it's related.
Check the sheet metal inspection cover on the transmission to make sure it isn't contacting the flexplate bolts or something. The torque converter balloons slightly as power is put through it, and it may be enough to contact the cover under load.
Another thing to check is that the engine and trans are in the same position as before the work. If things shifted slightly during the installation, you may be getting interference with other components. May have even damaged an engine mount, that sort of thing.
See if you can duplicate the noise with the trans in gear, foot on the brake and a bit of load on the engine. Careful!

had the same thing happen on a 65 mustang.
try tightening the torque converter bolts.
that's what my noise was

I had this noise prior to the trans rebuild. I was hoping that the rebuild would take care of it but it didn't. I asked the transmission shop if they new what was making the noise, they said the trans wouldn't make a noise like that...no moving parts.

Make a quick check to see if it's a heat riser valve in the right manifold. I would think it would rattle in park too as you rev but who knows. They were nortorious for rattling and a quick fix.

Did you check the torque on the flex plate bolts before you installed the tranny?

I will check the heat riser. Although I don't think that's it. It is more of a metal rubbing on metal sound.

No I didn't check the torque on the flex plate as I had the job done at a shop.

Rubbing or rattle? Two different sounds with different causes. Rattle...flexplate cover, starter heat shield, loose fan bolts, heat riser, loose air cleaner nut or air cleaner not centered on carb, loose bolts on core support, fender braces, or elsewhere in compartment.

Timing chain is easy to diagnose by rotating crankshaft first in one direction then the other, noting how far you can turn without disturbing the position of the rotor. It would have to be pretty loose to slap the cover.

TOHCan's suggestions will help isolate engine/drivetrain from body noises.

Sorry for the vague description. I have listened to it and listened to it and can't figure it out, let alone try to describe it in a email. You have given me some very good suggestions, which I will try.

Thanks everyone.

Don

A friend of mine had a metal on metal sound also. Turned out a bolt holding his console to the floor was too long and hit the driveshaft when he applied throttle or had a passenger in the car. Not enough to score the shaft, but enough to drive him crazy.