my highs are making a really annoying whistling sound, and ive been thinking its engine noise, because the higher the enginge is revved, the higher the whistle gets.
so someone told me in the sub forum that it may be that my amp housing isnt grounded, and i was wondering if anyone could tell me what it is exactly.
This refers to the earth cable. The person who advised you this thinks the problem is being cause because your earth cable isn't connected properly.
you need to double check this connection on the amp and on the car chasis. on the chasis make sure that you have scraped away any pain so that all you can see is silver and then bolt it down again.
You get that noise if there's a voltage difference bewteen your HU's ground and the amp's. Google "ground loop" and you'll find many solutions. Just want kind of amps are you using? Some cheap amps are proned to this problem, while more stable and higher quality amps have built in ground loop isolator. Double check your ground connections using DMM.