Eburnish

Hello, in the last month and a half I bought the flowmaster Super 40 muffler and had it put on professionally. Just in the last couple of weeks when the jeep hits second gear and I stay steadily on the gas, there is an odd high pitched noise coming from the exhaust somewhere but not sure if it could be the muffler that was just put on. Mind you I only bought the jeep about a week or two before I had the muffler put in and I never noticed that sound. A friend of mine doesn't think it's the muffler but it would make most sense... Any thoughts?