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High Speed Humming/Moaning

Hi All,
I am about to buy a 2003 Subaru Legacy Outback Sedan H6-3.0. When it gets up to about 40-45mph it begins a low hum or moan and gets louder as it goes faster. At freeway cruising speed the whole car is moaning with what I would describe as a mild vibration. My first inclination is to think it is a wheel bearing issue.My wife thinks it might be a tire issue. She said she remembers the friend that we might buy it from saying that she had replaced two of the tires on it. I know that on all wheel drive vehicles you have to replace all 4 tires at the same time. Could that cause a sound like this? If so how long might it take to cause damage to the drive system with different size tires?
Thanks for any feedback!

Buying a used car from friends or relatives is a good way to ruin the relationship. This is an old car with a strange sound. Two tires were replaced vs all four which can cause havoc with an AWD system. Someone posted here that used cars are like public transit buses, let this one go and another one will come by in a little while.

I agree, this could be a lot of things and very expensive. Take it to a good mechanic.

13 year old vehicle could have all kinds of problems and the fact that only two tires had been replaced would keep me from buying it. You might tell the owner if they can solve the moaning problem you can talk again.

If all the tire sizes are the same, and just 2 of them have more wear than the other two, I doubt that is the cause. Something about the tires themselves could be; maybe they are noisy, an aggressive tread design for example. The symptom sounds very much like the one I had on my VW Rabbit, which turned out to be both front wheel bearings were bad. That’d be my guess.

Thanks for the comments! Living in snow country, I’m familiar with the sound of aggressive tread. This sounds more internal and unfamiliar. Good mechanic is my next course of action. I’ll post back when I know more.