Tuba noise from engine compartment

Have a weird issue that has been going on for over a year. My car makes a sound like a tuba randomly. I never know when it is going to happen it just does. Sometimes I can get it to stop by revving the engine and sometimes I have to shut the car off to make it stop. It happens when it is cold out, warm out, in park, in drive, first thing in the morning or in the afternoon. Sometimes it does not happen for weeks then starts up again. I have tried cleaning the IAC and it still happens. Help!