Dryer makes rumbling sound when starts

When you start the dryer it sounds ok for the first 5 seconds and then
it starts to rumble and then it strarts to make like a popping sound, like
popcorn popping. It also makes that rumbling sound when you stop
the dryer. Another thing I can do but I am not sure
you are suppose to be able to do this is that I can spin the drum from
inside the dryer. When the dryer is off I can reach my hand in and then rotate the drum. Any help would be appreciated. Thanks
Scottnvegas