For the last several weeks, maybe 20-30 loads or so, when I start my
15-year-old Maytag dryer, it has made a single thump as it starts.

The noise is exactly like the noise you get if you start a dryer with
(for example) a sneaker or a really heavy wet towel or sweater in it.
Except I get it even with loads containing no heavy items, and even if
I start the dryer when empty.

No performance issues with the dryer. All loads dry nicely and in the
usual expected amount of time.

I usually notice odd things right away and don't ever remember hearing
this noise until recently. Any ideas what it is?