Weird Sound from Engine....?

Ok, I know the subject title doesn't make real sense, but I'll try to explain it. Just this weekend I got two 12" subwoofers and a 500 watt amplifier and a CD Player Reciever hooked up into my car (I own a 2002 Mitsubishi Lancer ES Manual Transmission), and it looks and sounds great. However, the other day I was driving and my girlfriend was sitting in the back and everytime I am speeding up there is a low sound that sounds like a hot tea-pot giving off steam. It's not as high pitched, or loud, but its somewhat like it but almost near silent. You have to really put your focus onto hearing it. *Note that we noticed this after the system was hooked up.

Now, what I'm thinking is that maybe it's the battery being stressed?? I noticed that when the light in the car is turned on, whenever the bass bumps, the light somewhat dims then goes back to fully lit until the next bass bump. I just got a capacitor from a friend, but I dont know if I want to spend the $ to get it installed if it wont solve the problem.

I know this wasn't much of a good explanation but it's what's happening.

Now that this has been stated: If anybody knows how to install a capacitor to this car, please help me out! Thanks

nope, its a ground.   check the grounds to the stereo and to the amp... one of them is not good.  you may need a larger ground or a more solid ground.

and by putting in the capacitor you'll reduce the dim when it the subs hit, and save your alternator and battery life as well.

Ok, thanks a lot guys. I'll be calling up the shop that did it for me tomorrow.