Tonights the coldest my truck has seen since I've owned it. (Only 26 degrees, but cold as hell for me lol) started up as normal and idled at about 850rpms for a few minutes then jumped to 1000 and held steady. Then I noticed a faint hissing, which would be the ebpv closing right? After a couple minutes of that it got a real deep, mellow, resonate tone in the exhaust note. Finally left and it still had the resonating mellow tone, like a gasser cruising. Could this be caused by the ebpv being closed?