Knocking noise coming from belt area. when belt

I do think that loose key and keyway could cause the knock. It would also explain the loss of power, due to the engine timing not being spot on.
A knocking big end or main bearing would not improve at higher revs, it would get worse.

Act soon before the keyway in the crankshaft itself is too badly damaged, by removing the crankshaft pulley, lower timing belt cover, and belt guide. You can then get a precise idea of what exactly is worn. This may assist

http://www.turboninjas.com/camry/eg1.pdf

You may actually have to find an oversize key, for a neat fit in the crankshaft. It may be necessary to file or grind it to fit. Engineering supply houses stock them, eg

http://engtooling.com/webshop2/

IS IT SUPER CHARGED ?
TRY REMOVING DRIVE BELT(S) AND ONLY BRIEFLY RUN TO SEE IF NOISE
GOES AWAY 2 BELTS ON S/C ENGINE AND ONLY 1 ON REGULAR ENGINE

While the engine is cold, remove the serpentine belt and start the engine and see if the knock goes away. If the knock goes away, then you might have a bad belt pulley. Good luck, Gary

You need to remove the belt and check each pulley for looseness and noise when turning by hand. You can also start the engine without the belt on to see if the noise is coming from the engine itself.

the damper could be bad if it's the type with the rubber inbetween the pully and the the rest of the damper. check that and see if it is separated. if not looks like your crankshaft bearings could be bad

Need more info, what kind of noise. How many miles are on your car.What engine is in your car. If the noise sounds like an engine knock, have someone take the tension off of the belt while you check to see if Harmonic balancer moves back and forth. If it does it needs to be replaced. It will sound like an engine knock when bad. When this 1st starts to go bad it is only there part of the time. If it is this, it will eventually be there all the time and get much worse.

you should check timinh tensioner. thats probably the problem...

yoy could have a bad t belt tensioner or bad t belt pulley. get this looked into right away. you have what is known as an inteference engine. if you lose that belt for any reason, it WILL drop a valve, and your're looking at a lot of money for an job.

check your timing chain or belt might be loose and or your belt tensioner is not working

The top section of the engine is called a '**** shell' there are baffles inside tha break away and knock. If you press down on this plastic section and the knock goes away. then you have to replace it. It unbolts usinh\g 10mm socket. You need a short one and a long one. also remove the IAC solenoid above the throttle body. **** shell is a dealer item only. also replace the gasket seal.
If the **** shell does'nt come, out gently use a pry bar between the throttle body and the cowl area, for more clearance.