Cleverrya

I just bought a used a4 and it has a whine coming from the front, it only happens when my foot is on the gas, doesn't happen when I'm going downhill, doesn't happen in neutral, gets louder as you accelerate and peaks around 45-50mph. I'm just trying to get a list of possible causes.

John535is

Does the sound change with speed or with revs?
If its only making the noise on power acceleration I would be looking at the belt tensioner on the timing belt then probably the bearing on your alternator. Do you know when the TB was replaced? I have seen some belts when they start to come apart make a whining sound as they spin faster but that should happen even in neutral if you rev up. Just to start the thoughts.

Cleverrya

The sound does change with speed. The sound happens when I am accelerating or holding a constant speed but doesn't happen when I rev in neutral.

John535is

So if it does not change with engine revs but with speed it must be transmission, axle or wheel related. Start with checking CV's, check for broken boots, leaky mess around the boots or if one or more boots look like they have been replaced recently. Some people will get a cheap job by just replacing a torn boot over a CV that is failing then sell the car. Next you are looking for a dry or collapsed wheel bearing. Check the tires, take them off and make sure one is not distorted or torn on the inside, check the tread is not delaminating, then check the transmission.