Hissing Coming From Beneath Dash (Heater Core?)

Hello, I have looked at other threads and have yet to find an accurate description of what I am Experiencing. I have a 2006 Double Cab v6. About 4 years ago I got the AC Compressor replaced because the clutch on the old assembly quit. Ever since that day I have heard a small hiss coming from beneath the dash. Anyways Heat and AC both Work Great but now the Hiss is noticeably Loud Beneath the dash. My first Assumption is something in the heater core Because with Defrosters on that sucker will ramp up in hiss then stop ramp up in hiss then stop, etc. Should I be worried or crank up the tunes? I just am aware that the heater cores are a PIA. Thanks

 

Does it do it only with the AC on...if so could be a expansion valve or low on freon