I have lost significant water pressure in my Pfister WKP-600Y faucet and now it whistles or hisses when it switches to hot water. What is causing this and how do you fix it?

Sounds like you've got some trash in the cartridge inlet ports.
Shut the water off and open up the valve to remove the cartridge. Check and clear the inlet ports. Now place a pan over the valve and turn the water back on for a moment to flush out the supplies. Reassemble and test. Good luck, Tom