pittsteel

I have been researching and investigating a simuliar rattling sound when DFM kicks in on my 2021 Yukon AT4 w/ the 5.3L. Every time there is a load on the enigine (going up hill or even flat) and I have a little toe on the gas I get it. What I've found so far is there is a exhaust valve mid-way back just before the muffler that isn't working correctly. The value from my understanding and reading helps control the back pressure in the exhaust when DFM is on. This valve is a spring loaded valve that apparetly weakens over time. The flapper inside the exhaust (cant't see it) sounds loose and potentially could cause a fluttering rattle sound that resonates up the exhaust chain. I locked my valve fully open and it killed some of the rattling, but not all of it. Now that the valve is fully open I get a rubbling sound kinda like opening the back window of a car and the pressure inside is fluxating. I suspect this is from the valve being disabled and not working to kill back pressure. I'd rather hear that than the rattle. I also tested this by diabling DFM manually without the need of a tuner. You can take your vehicle and put into manual shift mode. On my Yukon I push the 'L' button on my shifter. Then using the - and + buttons I dial the number up to 9, telling the truck it's max gear and to only use 9 out of the 10 gears. With it in this mode DFM will never come on. If you hit + again and set it to 10 DFM will start kicking in again but only when in 10th gear. To take it out of manual shift just hit the D button to put it back into automatic. Engine response is greatly improved when in maunal mode so I may have some DFM issue going on. When in DFM my engine is a slug. It stumbles and hesitates coming out of 4 cycle mode. Some history with my vehicle. I had both banks of lifters replaced at 13K miles due to lifter failure. This rattle started this year around 30K miles. I'm currently at 32K. It does suck to pay the amount we pay for these trucks and have so many issues. I have an appoint setup towards the end of Nov for my dealer to troubleshoot. But, as you've all have seen - diagnosing a rattle isn't fun! matt grz, not sure if your exhaust update removed this OEM flapper valve. I think most cat back exhaust upgrades do not. You may want to check it out.