1999 Jeep GC Mechanic Stumper - LOUD at startup

Hi there,

I have a 1999 Jeep Grand Cherokee. I've had this problem for over a year, and no mechanic had any clue what the problem was.

Every time I start my Jeep in the morning, the engine is SUPER loud. When I rev, it gets even LOUDER. The only way to bring the engine noise down is if I rev over 4500rpm. As soon as I do this, the engine sounds nice and quiet like it should.

If I have the car running for a while - I can turn the ignition off, and turn back on with the normal quiet start sound. If it's left go for a few hours - the Loud noise will return on startup.

I have taken this to 3 mechanics - with no answers.
Actually, 1 mechanic didn't even acknowledge this was a problem.

When I took it to a Jeep dealer orginally they said it was the fan clutch. I replaced it, and it orginally fixed the noise. Then after a few months, the loud start up noise came back. I had it replaced yet AGAIN, but this time it didn't solve it.

Any answers mr. Mechanic?

Thank you so much in advance,
Aris