One last thing to check...

Using a 1/8"-headphone-jack-to-stereo-RCA Y adapter, plug a pair of small earbud headphones (must be small earbuds, not large over the ear headphones) directly into the line level RCA output jacks on the back of the player itself. Plug a power supply directly into the player's AC adapter connector. You are using just the player itself, bypassing any kind of a sled or dock, and bypassing the sled docking connector completely.

See if the problem is still there when listening through the headphones. You may have to turn up the player's volume louder to be able to hear clearly.

This won't necessarily rule out, or rule in, a general hardware failure, however it may narrow down the *location* of a potential fault.

(Oh, and just for the heck of it, you're sure there isn't a loose screw inside the player?)