Squeak problems with Mazda3

[RobertInOntario]

Here I am with another car question (don't worry, I'm still model railroading!) … Since the beginning of October, our 2011 Mazda3 (with 200,000 km) has been making a squeaking/creaking noise when travelling straight at low speeds. This noise is intermittent and seems to be worse after we’ve used the car for 30-60 minutes. 

It also seems to disappear once the cold weather arrives. Just recently, winter has suddenly hit us and the temps have dropped to around freezing or well below freezing. So now the car is not squeaking much if at all.

When the noise does happen, however, it's so annoying, that it’s stressful and distracting. Sounds like the car is going to fall apart.

So far, I’ve taken it to 4 different garages and all say all of the suspension elements are fine:
• local garage (whom I know & trust) — couldn’t find anything wrong, assured me it was OK.
• Midas — same as above.
• Mazda Dealer — also couldn’t find anything obvious, but said that an engine mount and a tranny mount needed replacing. I had the engine mount replaced at my local garage (cost $174). Squeak is a little better but still there.
• Muffler Shop (whom I trust) — also couldn’t find anything wrong but they suggested replacing the muffler (they said, once the muffler gets hot after use, it shifts around).

I think my best course of action is to go back to my local garage and get them to replace the failing transmission mount. 

Open to feedback & ideas!
Thanks in advance.

Robert, the transmission mount could be the cause and if it is bad it should be replaced before damage occurs from the shifting around the engine assembly could do if it completely fails. It could also be as simple as a fan belt.  Intermittent noises are expensive and hard to find. 
Charlie

Thanks, Charlie! It's booked in for early December (at my local garage) which is the earliest appointment they had.