I wouldn't change out the contactor unless just for the heck of it. Especially if the noise stops when you turn off the breaker for the outdoor system (while leaving the air handler breaker on) with the heat "on".
In the past three days, working in the Huntsville area we have took or ran 5-10 calls a day with ice building up on fan blades from the misty rain or water dripping off of roofs, that'll definately cause some noise issues and make a fan run out of balance. If you see no ice on the fan blade and everything seems to be running smooth it's probably just due to the fact that it's past its structural balance point and running at the max that it can. When the weather pushes units to their max it's not uncommon for them to make a little more noise, especially on builder grade equipment. Also as several others stated, lack of routine maintenance can also make things a little worst in the peak seasons.
I'm a service tech at a large reputable company in Huntsville. If you need someone to come out and look at it, shoot me a pm with a ph number and I can get you some info.
