Minimal RPM pulse + deep grumble noise for a second

Josey Pukrop · July 28, 2022

2014 Silverado 1500 LT with Rally Package, 210k miles

About 3 weeks ago the truck started to have a very minimal, very intermittent pulsing in RPM, most noticeable around 45-50mph. The tach would bounce maybe 50-200 RPM at a time (primarily in the 1k-3k RPM range) and only last a couple of seconds. In the past week at highway speeds (65-70mph) I've been getting a very deep rumble/thrumming/groaning noise (or like a vibration, but not the kind that you feel, just hear) that lasts for a second or two, repeats a time or three and goes away. Frequency of both the RPM pulse and the rumble have been increasing where it'll happen almost every time I drive (primarily short trips- 5-10 miles at a time). Seems to happen a bit more when going up a hill, but will occur when coasting or accelerating.

Over the course of the past 6 months I've had maybe 6 times when I'd get a check engine light to flash. It was always going over a mountain pass or hauling a trailer with tow/haul engaged. Would last for 30-90 seconds max and then go away- either on it's own or if I restarted the truck. Finally got a code reading for a cylinder 5 misfire and found that I had a cracked spark plug wire. Replaced that and haven't had the light flash again yet. Potentially related to the RPM pulse, potentially resolved by replacing the spark plug wire.

Spark plugs and wires were all changed about 35k ago, though I've hauled livestock a decent amount, so could be that they are due again. I pulled the plug on cylinder 5 and it was still in great shape. When I had the misfire light I swapped the engine coil on cylinders 3 and 5, but that didn't produce any change in performance. Any thoughts on easy things I can give a try? My best leads have been that it could be the coils or something with the fuel injectors. I do have a bluetooth scanner and torque pro, but haven't figured out how to pull misfire data on it. I've been waiting for a new engine code, but haven't gotten anything, so I don't have any good leads.

TIA!

CamGTP · July 28, 2022

The rpm flutter at those speeds and for the short period time make me want to think torque converter. The stock torque converter at that many miles is getting pretty dicey if you ask me, they are known to fail at less miles than you have.

Most code readers can't see transmission codes unless it's pretty fancy and the better scan tools would let you watch live data to check on the functioning of the torque converter. If there is a lot of extra clutch slip, it's on it's way out.