Every time I turn on my rear defroster it makes an annoying noise. Even when the mode is on the foot well and front defroster. Not really sure if warming up or something with my heater is the problem. I do wait at least 2 minutes to warm up before I head out.
If you mean the rear defrosting wires in your rear window, you should be able to turn it on immediately as it's electrically based. If it's making a noise, I suspect it's the circuit breaker that automatically turns it off based on heat, and you should look into replacing it. What make/model is it?
If, alternatively, you have some sort of auxiliary/rear blower, the noise could be the auxiliary/rear blower motor as the bearings are prone to failure.
From the comments, it sounds like this is not a motor issue and varies with engine RPM, which indicates to me that you have a loose belt or worn belt tensioner (causing the loose belt) to the alternator, or an alternator with a bad bearing.