1999 Mercury Cougar humming

I had that humming noise go on for quite awhile. It's the ERG or EGR valve getting stuck. It's a valve in your exhaust system so when you give it more gas it builds enough pressure to go ahead and pop it open. I've had it replaced twice on my car so far but the second one seems to be holding up ok. If you don't get it replaced and it really gets stuck it apparently could back up your engine and cause some major problems.