Hello. I have a 2017 Sierra 1500 with 5.3 with 77k miles. I have noticed a rattling noise when accelerating uphill and trailering, most often at highway speeds although I have heard it at lower speeds. I thought maybe it was spark knock so I changed the spark plugs and ran a full tank of premium ethanol free gas through it with no luck. I currently don't have any codes but have had transmission problems to include a p700 and p701. The dealership flushed and changed the transmission fluid but that hasn't helped. I inspected and have not found any loose or missing bolts in the engine bay. I am concerned as my warranty is coming to an end and even though I show videos of the noise to the mechanics they say they can't replicate the engine rattle. Any help is greatly appreciated.
Rattling is frequently caused by deteriorating heat shields in the exhaust system.
I should add that when the rattling occurs, the motor doesn't seem to want to down shift. Seems like I'm pressing excessively on the gas pedal before if finally shifts
You could use a noise-finding device like the ones Scotty has demonstrated to try zeroing in on the source of the rattle.
https://www.youtube.com/watch?v=Wy5nqh3LSs4
so I changed the spark plugs and ran a full tank of premium ethanol free gas through it
You wish! Is the rattle coming from the engine bay area?
The dealership flushed and changed the transmission fluid
Flushing can create transmission problems. The codes P0700 & P701 indicate a problem 'relating to' the Transmission Control Module. Possible cause.. The TCM suffers from poor connectivity. If there's a bad or loose connection at the TCM , your codes will pop. Try carefully checking all the connections at the TCM. A loose pin can do this.
6 or 8 speed?
These might explain better P0700 on a GMC 1500 :Loose TCM harness
Hi yes the rattling sounds like it's coming from the front driver's side of the engine bay. It has the terrible 8 speed transmission. I had it in for hard, clunky shifts and rather than take it apart, they flushed and refilled with whatever gm says "fixes" the issue.
I would also add that the rattling only really occurs after I've driven for a while, about 20 minutes or so. I thought maybe a leak of some sort at first but given everything is heated and "swelled" it's not likely. I've also noticed a slightly heavier exhaust smell.
only really occurs after I've driven for a while, about 20 minutes or so.
The terrible 8 speed transmissions is plenty warm after 20 minutes. I would make sure the transmission is cooling properly, make sure there's no weak/failing connection in that harness. The reason I suggest looking at the TCM connections is it's a common issue, but can be a low cost repair.
Have it scanned with a high end, bi-directional scan tool - those 8 speeds are 'very' computer driven.
Thank you for the feedback. I just took it to my buddy who has a snap-on tester and didn't have time to run it around. Did find out there's a p0711 code in there but no check engine light. I'm just gonna make an appointment to take in for warranty. Hopefully they can address the transmission and rattle noise. Once they do that I think ill just have it tuned to disable the afm feature as I think that is garbage and probably doing more harm than good.
disable the afm feature
Good move. I've never viewed cylinder deactivation as beneficial. (long term it's costly)
You have a warranty & a time constraint. Take advantage of it while/if you can. Might be able to get things sorted at minimal costs.
Hey, @Hixster. Well GM ended up replacing the Trans temp sensor. Shifts a ton better now but they didn't do anything with the rattling from the engine. Told me they "tested my fuel" and had 10 percent alcohol and that "I should run premium gas." I asked them how they determined it wasn't premium and they said "because my alcohol content wasn't right." I replied that it's standard to have between 10-15 percent in all grades of fuel... anyways... they charged me 200 dollars to not do anything with the car other than tell me to run premium.... WHICH WAS ALREADY IN IT! I swear the noise from the engine bay is worse now than it was before and the "spark knock" rattle happens more often now than it did. Idk maybe I should get one of the chassis ear testers and see if I can hear where the noise is coming from.