Have a problem that might get more serious later on and is not an emergency right now although it isn't exactly maybe safe either.

I have a specialized stout rear wheel that sometimes pops loose at the hub when pedaling and then is fine for some time before happening again.

It happens maybe once or twice every 10 miles or so and it wouldn't be a good thing if I were standing on the pedals when that happens.

My question is if anyone knows if the Specialized wheels have hubs that can be replaced or is it only one per wheel.

Also if I slowly backpedal with my hand I can feel some slight rumbling through the pedal and frame.

Obviously you aren't equipped to deal with this yourself, so why not take it to your Specialized shop and ask them to look at it?

'Pops loose at the hub'? So many things to inspect first before even considering the hub itself. Can you post a picture of the wheel in the dropouts and the quick release skewer you are using?

Yes, re describe the problem without using the term "pops loose". Andy

My guess and it is only a guess is the pawls are mucked up in the freewheel. Maybe removing the freewheel body flushing it and re-lubing it might help. Worst case scenario is rebuilding your rear wheel.

It might be dirt in the freewheel, another suggested this. It is a single speed setup and so it's fairly certain that it's the freewheel. It pops before engaging I mean. It can happen after backpedaling or coasting but may have happened a few times even when pedaling.

Can the freewheel or freewheel pawls wear down?.

if it's dirty would that also cause damage?.

I think it also tightens up a little before engaging as well, so a piece of sand or something might get in the way of the pawls and slip out.

also isn't cleaning the hub and putting fresh lube in the same as rebuilding the wheel?.