(P)245/70R16 tires on a 2003 Trailblazer LS - too large?

The "P" is in parentheses because only the front tires have the "P" before the size number, the 2 back tires are just 245/70R16. I was told that these tires are too large for this vehicle, and I'm thinking maybe this is the cause of the sound I hear when on the highway, sort of a high pitched "whoop-whoop" that seems to becoming from the rear. The dealer went so far as to change the wheel bearings, and was told that it was the tires, that they are so wide. Should I start saving for new tires because of this?
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