What is the model number of this dryer - Would like to see a breakdown of parts and how it is put together.....
The noise sounds like it would have to come from a moving part.....
As mentioned by others
Idler Pulley
Drum Rollers
Motor
Could also be coming from the drum itself - if a drum felt is worn out....
If there is some type of rear bearing, like a Frigidaire or GE type.....
A model number with a breakdown of how this unit is put together will help....
Then a disassembly and inspection of the parts should determine the problem!