I know, this is an old topic, ...but I've been just putting up with the noise for the past 2+ years. I did take another look at this after the last post, but unfortunately, once I pulled the dryer out, it wouldn't make the noise when I had the top and front off, so I was unable to verify the source of the noise. Once I reinstalled it, the noise returned and I just gave up.
The noise continued to annoy me every time we used the dryer and I recently decided to try (again) to fix this so I went ahead and ordered a new Heater Assembly to have on hand. So I decided today was the day and pulled the dryer out. Popped the top and front covers, and shorted the door switch so I could run it. Thankfully, the noise was there this time so I knew I would have a better chance of figuring this out. At first, I thought the noise was coming from the heater assembly, but I when I was poking things with a wooden ruler, I couldn't get the noise to stop or the 'pitch' to change, so I kept looking. I removed the dryer lint duct to get a better view inside and that's when I spotted the metal Spring for the Idler Pulley. Sure enough, when I poked the spring with my ruler, the noise changed and I was eventually able to get the noise to stop.
It seems the noise starting up after the Heater replacement was just a coincidence or possibly a result of the spring moving slightly as I moved the dryer around. The spring appeared to be resting against the edge of the sheet metal bracket for idler. Although moving the spring away from the bracket stopped the noise, I didn't want to leave it like that and just hope that it didn't return, so I decided to secure a piece of 'insulating material' between the spring and the bracket with a cable-tie.
So far, so good! I don't know how (un)common this issue, but I wanted to share what I found just in case anyone else runs into this.....just sorry it took me 2 1/2 years to figure it out!