I just bought a 2001 Mc ss and it has a whistle noise from the motor at idle but when I rev it up it stops

 

 

It might be a tensioner bearing. With the engine off, try spraying a bit of wd40 or pb blaster around the bolt/washer on each of the tensioner and idler pulleys, then start the car and see what it does.

Does it run well? Are there any check engine lights?

 

I put a new belt tensioner on it it still does it. It only does it when it gets hot and it only does it at idle there are some codes and it runs ok

 

What are the codes? Those might give you some indication of what is going on...

 

check the airbox and make sure the air filter is in place properly. and
make sure there are no bird or mouse nests in the airbox.

 

Sounds like a potential vacuum leak to me.

 

I'm gonna suspect as mentioned a bearing in a pulley( could be an idler pulley, alternator, waterpump) or could even be as simple as the serpentine belt. funny coincidence but a friend drove his wifes car just Friday to ask me what was goin on, it was squeaking from the engine bay. by time I went to look at it the engine was cool, started it up and it sounded quiet as a mouse. gave it a few minutes to heat up and then the squeeking began. turned out to be waterpump bearing on his.
I however when I got my 06 monte also had a squeak but only at startup, would go away once engine warmed up. turned out to be just an aged belt. the belt looked good visually but I put a new goodyear belt on it and all was great.

 

The crankshaft position sensor and the fan

 

It still made the noise with the belt off and it only does it when it gets up to running temperature and it stops when I rev the motor to 3000 rpms