metallic tick/knock driver side wheel well

[sterling226]

so i will try to explain this best i can.. 2017 gmc 2500 L5P-50,000 miles

so i drove to idaho for vacation last week and heading home i started my truck one morning an normal idle rpm is usually around 500-550 ish i believe an (truck was cold) and after about 3 minutes the idle bogged down to bout 450 rpm went back to normal. after 20-30 seconds the idle went up to around 700 rpm then went back to normal did this process for a out 5 minutes then went away an started driving an seem to hear a metallic knock/tick that can only be heaard inside the cab or under driver wheel well and only while under load (drive, reverse NOT in nuetral). an can be heard very well if coasting down a hill under 20mph and while accelerating, slightly heard at all other cruising speeds. do feel a slight power loss but not a lot, no engine light/codes fluids are all good, everything has been checked. had a mechanic (not gm dealer) listened to it an said sounds like a lifter or a exhaust leak.the idle issues only happened once.

anyone else run in to something like this?