Hi I installed a single handle glacier bay shower and tub faucet and when I turn it on all the way to hot it runs fine but when I turn it down to adjust the temp there is a rapping or knocking sound coming out of the valve. And it is also hard to regulate the temp it gets cool and I turn it up a little bit and it becomes scolding hot then I turn it down just a hair and it get cool again.
Any suggestions on what is wrong?

Sounds like you might have dislodged some mineral build up in the pipe walls when you installed then valkve.
Try this!
Shut then water off, open up the valve and remove the cartridge.
Check the cartridge inlet ports for trash and debris. Now turn the water back on to flush out the supplies and open valve body. Reassemble and test. Better now? Let me know, Tom

Thanks tom I'll check that out and get back to you.
Jay