From the time when you put in drive and release the brake, there is a light hum. Once at 35 or 55 you can here a grind and hum, once you slightly press the gas it lightens, but if you push the gas it gets louder, would this be diff or wheel. I had my son ride in the back seat and he hears it coming from right side not center of car...any advise would be great

rear wheel bearing.

 

it could be rust rubbing against a pad or a pad hanging on the pad clips or slide pins - so inspect your brake hardware.

 

if it's been doing it awhile the effected pad and rotor side will start to show evidence as well - uneven wear or corroded spots on rotor from pad not keeping it clean.

 

subaru rear diff failure is almost unheard of.

The wheel bearing noise does not usually change with throttle, only speed and when turning, but there are no absolutes. Of course it might be very bad and the wheel might be angling a bit with power.

 

The noise can be difficult to hear spinning the wheel by hand and with no weight on the wheel. Best bet is to get both back wheels up, check to see if either is loose, then spin the wheel by hand and listen for noise. Putting a long screwdriver or long socket extension from your ear to the strut spring can magnify a small noise.

 

You may need to pull the wheels and back the brake pads off the rotor to get it to spin quietly and/or freely. Do this after trying it once so you don't miss the chance to hear if the brakes are making the noise.

 

It could also be a bad control-arm bushing, trans bushing, or rear diff bushing, allowing noisy metal-metal contact when under load.