My 2018 Silverado 1500 5.3l 6sp is making a screeching sound in V4 mode. I HAVE AN AFTERMARKET EXHAUST so it is not the flapper valve issue. Best way to describe it is when in v4 mode and there is an increase in load on the engine (slight uphill) it starts to squeal almost like a bad serpentine belt. It sounds like chains dragging on the ground combined with a belt screaming. Not sure if this is a lifter going bad, something vibrating in the exhaust section, or something different. I’ve pressure washed the underside of truck to see if there were any rocks causing a weird vibration, but I’m thinking it’s an engine issue. Goes away the second it goes back into v8 mode, or the load decreases in v4 and the truck is coasting. Any ideas??
