I have a 2004 Infiniti g35x and I recently changed the front breaks. A few days later I hear a knocking sound coming from the passenger side rear. It happens when I let go of the gas and gets louder when I step on the breaks. Also there's a squealing noise from the driver side rear. Anyway I'm more concerned from the knocking as it also kind of shakes the car when I go 65-70 mph. Also the car feels a bit heavier when I'm trying to accelerate. Please help don't know what it is.

Better take apart and recheck what you did.
If it started right after the work, its pretty logical that its related to your work.
Did you do anything to the rear?

No didn't touch the back.

When was the last time you checked the pads and rotors on the back?

Maybe it just sounds like its from the back.