loud knocking sound around 4500 rpm's....

i don't know what kind noise your hearing but, my car used to do the same kind of sound and i found out that my car was pinging. i was using the cheep gas so i put a higher octane in and it stopped.

my symptoms:
5000rpms it made the noise and lost some power.
if you chug it down low in third gear it dose the some noise.

this could be way off but it was worth a say.