2002 Camry weird engine noise. Help!

I have 4 cylinder 2002 Camry. It has about 39000 miles. A while ago it started to make weird engine noise. Everytime I accelerate from stop, it makes this 'grrrrr' noise for about 2-3 sec. Longer the stop, like red light, clear the noise is. It only does it from stop to go. The performance doesn't seem to suffer but the noise is getting clearer and it happens every single time I step on my gas from stop. Any idea?

Do you know what pinging sounds like? Could be that.

My car started doing that in reverse after I got my front engine mount replaced. My exhaust system touches the frame when the engine torques in reverse, making a "grrrr" kind of sound. You can notice it resonating through the body of the car. Could bad motor mounts or an exhaust system out of "alignment" be a potential problem?

I have heard that Noise on my 4Cyl also but only when its cold out in the winter but goes away whenthe engine warms up? I will have to wait till winter arrives again to see if it has got worse