so i hooked up my system with the four way amp. Now, theres is a whistling sound coming from one of the speakers more than the others. It is very faint you can really hear it when the volume get tuned up but you can when its down or on mute. Any body know how to diagnosis this?

 

check ur connections and amp ground

 

is the sound proportional to your engine rpm's?

as stated above, check your grounds including the headunit.

 

yes it is

 

You have "alternator noise", there are a number of possible things wrong, a ground loop is probably the number one cause of alt. noise, number two would be running the RCAs with the amps power cable, or along with the cars wire harness.

To be more help I need..

MM&Y of car?
Makes and models of HU, amp(s), speakers and subs.
What kind of RCA leads, where and how they are run?
What kind and gauge of power and ground, and where and how is it run?
How long is the ground lead and where and how is it grounded?
Where is the amp mounted, is the amp chassis grounded to the cars chassis with the mounting screws?
Does the noise get louder if you turn things on, like head lights, high beam, rear window defog? 94