Delta shower troubleshoot

[Oh-no]

I have a Delta single-handle shower that's probably 11 years old (I don't know the model number, but it does have the scald guard feature). The water pressure going to the shower head is inconsistent and the shower itself will sometimes make a whiny noise. I'm trying to figure out what's wrong, but I have limited plumbing experience. The hot and cold seem to be working fine. But, you'll turn on the water and the pressure is very soft. Turn it off and then back on a couple of times and the pressure increases. Is this a valve issue? Cartridge? Other?

[Norcal]

Probably the cartridge.