2005 Mazda 3 Low RPM Noise HELP
 

So when I'm sitting in park, usually runs at around 900 rpm's which is normal, but when I am sitting still (like at a red light) in drive my car drops down to around 600 to 700 rpm's and you can hear a sort of loud vibrating noise that seems to be coming from the passenger front side. It's a noise thats hard to explain, shakes the car just slightly. Any solution?