It can be a few things.
The drum that spins is supported by a single bearing in the middle center in the back. That can be faulty.
The drum sits on round nylon bearings to support it when it turns and they can be worn out causing the bearings to make the noise.
The felt ring that surrounds the door that makes the seal between the drum and the front of the cabinet has worn out and the drum is wearing on the cabinet itself.
If the noise goes away shortly after the drum begins to spin and it gets longer or worse when the laundry is inside putting weight on the drum you can also add fan belt is wearing and causing this noise.
To determine if its worth it or not to fix, one must consider the age of the unit. While parts are still available for most dryers, once bearings start to go, its pretty much worn out and time to replace it.
Thank you !! After reading this,I think it might be the fan belt. I have had it for 7 yrs,and it is used a lot.
Jeanette...I just replaced a unit for one of my clients, in their rental. The old unit was used when he got it and survived another 5-6 years. In this case the front of the machine was loose and a few parts were very damaged. I found a relatively new (used) model with a stainless drum on Craigs list for only $150... In this case it was a quick economical fix.
The people I got the dryer from had a stacked washer / dryer combo in which the washer died...they were replacing the "set" with a new set. The most basic new units I could find were in the 300 to 350 range.
Thank you,good info.