2004 Sub. w/ one heck of a knock

I own a :
2004 Suburban 2 wheel drive w/ tow pkg and 92k miles.

Heres my problem:

On occasion while driving the vehicle a heck of a loud knocking noise will start.
I can't really detect where it's coming from but it doesnt seem to be the engine.

When the knocking starts the vehicle is always in drive and moving. If I come to a complete stop or slow down to a crawl the knocking stops. If I increase my speed it will stop. If I increase or decrease my rpms the knock does not vary. It only last for 15-20 sec.s and it has not increased or decreased (loudness or rythm) since it first started.

I have checked the u-joints...had my mechanic(who has never heard this type knock b4) check it out....he checked the rear end and axle out no metal and all splines/gears were good. He also pulled the transmission pan (fluid was a dark w/ a bit of red tint) and there was a small amount of metal shavings on the magnet.

He says it has to be the transmission....(it does not slip)
What do you folks think?
If I have left out any crucial info please ask.

Thanks a mill for your time,

Ken Taylor

Welcome to AF, it is impossible in this environment to give you any meaningful advice with this type noise, you need to seek out a tech that has the experience to pinpoint this isuue, there are a mulitude of drivetrain possibilities that could cause this.