High Rpm grinding noise.
 

Hey all, like the title says. I’m running a 350/TH350 automatic in a 72 c10. I’ve been hearing a metal grinding noise, only when I get into high rpm/speed, around 55+. I never hear it unless it is around those speeds. Unsure as to where to start. Timing issue maybe? Anyone else experience this? Any help would be awesome, thanks all.

Re: High Rpm grinding noise.
 

Sorry, I have more questions than answers with that amount of info. You might try making a video recording for us and post it on youtube or other video site, if the noise can be heard on a recording. High speed grinding noise starts at high RPM of the engine or does the speed of the truck need to be high? Perhaps you can use 2nd gear to see the difference on when it starts. It would be good to know if it is engine/transmission/alternator/water pump RPM related, or truck speed related because it is somewhere else like brakes, U-joints, or bearings. Any difference when engine is not yet warmed vs. fully warmed up? Tell us lots about your truck, like do you have air conditioning, what kind of cooling fan is on your radiator, electric, or fixed, or clutch fan. What modifications have been done? Have you done anything to the truck that might have been the beginning of the symptom? Any seeping of coolant from the water pump shaft? How old are the tires? Have you checked condition of the differential gears and checked the oil in the differential?

Re: High Rpm grinding noise.
 

Thanks for the reply. If I had to narrow it down, it is speed related in third gear. For example, driving from 0-45 (or so) mph, no issues. But when I get up toward 50 mph +, there is a distinct grinding/clicking sound from the somewhere, and I haven’t been able to pinpoint where. It sounds similar to when the flexplate hits the starter when it isn’t shimmed properly. But I checked that, and it wasn’t the issue. As far as my truck, I have no mods. No A/C, no heater, radiator is new and so are the brakes. I hadn’t thought about U-Joints though, maybe I should check those for sure. Oh and I have no leaks anywhere and tires are good. I’ll try and take a video in the next couple of days, and post it on YouTube and check back. Thank you.

Re: High Rpm grinding noise.
 

Is the flywheel cracked around the crankshaft bolts?

Re: High Rpm grinding noise.
 

Just a simple guess before continuing to dive into the expensive diagnostics...

Have you lubricated the speedometer cable recently? Might be worth a shot if not.

Re: High Rpm grinding noise.
 

Quick easy test...disconnect the speedo from the tranny and go from there.