loud rattle or rub

my 91 c1500 is making a really loud rattling or metal on metal sound only on acceleration, especially under 20MPH. Had the U's and tranny checked they are fine, had new motor mounts put in since they were collapsed...the entire drive line has been check by pros and they insist that nothing is damaged. This just started a month or so ago. It comes and goes as well sometimes not for days. It almost sounds like the fly wheel hitting the cover or a fan blade of some sort. There are no loose lines either. I can not figure out what this is. It sounds like it's coming from around the tranny or u joints but they are fine. The mechanic said it was due to the collapsed motor mounts, but after they replaced them I drove away and the sound started again..this time different pitch and volume. It's also RPM related, it chages with the RPM's...and then sometimes it does not..

anyone have any ideas? 160,000 miles and except for this, it runs showroom new.

It could be a crack in your flywheel, My truck was making a similar noise around the bell housing, pulled the cover off and coudnt see anything, then pulled the trans out and it had a hairline crack in the flywheel, like you everything else had been checked and it all checked out ok. that might give you an idea anyway.

thanks for the idea. I thought it might be that as well since I hear some of the older chevy c/k flywheels crack from age and heat. If I don't replace it, do you think I will damage anything else? How long did you drive yours after the noise started and before you replaced the flywheel?

I've replaced 2 on my 88.

I drove mine for about 4 months before I did anything about it, but thats highway driving, I would go ahead and change it out, that way you want have to sweat it.