2004 Nissan Altima Thumping Sound

I have a 2005 2.5L A/T 73,000 mi and I just developed a similar problem in the front left wheel area. I have a friend of mine as a mechanic but another person in the shop looked at my car this time. I begin to get the "thump" or almost knocking sound around 40 mph and gets worse with speed. I thought it was the wheel balance or cupping but it feels and sounds more mechanical now. I feel the vibration all the way through the steering wheel. No sound on braking or acceleration though. I did just change the front brakes and rotate the tires. The mechanic wasn't sure exact cause but said maybe a axle going bad. I'm going to re-rotate the tires to see if it's a wheel problem. If sound still there, I'll then take off the brakes I just put on to reinstall them to see if that gets me anywhere. Let me know if you find anything else out.