Recently Ive noticed a low pitched, subtle hum coming from what I believe to be the rear end. It only happens when I am accelerating between 45 and 50 mph. It will stop when I take my foot off the gas or if I am below 45 or over 50 Mph. I had the oil changed in my rear differential about 40k ago and had a new pinion seal put on. I am not sure what this could be any ideas? Could it be that I just need a differential oil change? Could it be that I never noticed it before and it is nothing? It is not real pronounced and you would almost miss it if the radio was on. It also doesn't always do it if you are accelerating between 45 and 50 mph-its somewhat intermittent. Please help! Thanks.

 

My muffler is also on its last legs and has a few holes in it. Its starting to buzz and make noise. Could it be possible that it vibrates and hums a bit between 45-50 mph periodically?
Today it wasn't making any noises at those speeds so the noise is definitely intermittent. Please give me some insight. Thanks.