High pitch noise in driveline

[DT10]

Greetings,

I recently had the ring and pinion changed from 3.42 gears to 3.73 gears (2WD). Since the installation, I hear a high pitch (like a whistle) sound at certain speeds. The pitch does not change with speed. The sound is noticeable at 45 mph and 67 mph through 71 mph. The work was done by an experienced builder and synthetic lube was used. Any thoughts about this?

Thanks,

DT10

[chevy4x4trucks]

I'm not sure what the problem could be, but welcome to the site

[Shaners]

It's strange that the pitch doesn't change with speed.  All readings seem to be in OD.  Can you hear it if you leave your tranny in "3"?  This one seems a little perplexing...  I'll think about it for awhile...

[DT10]

Yes, I can hear it in 3 rd. The truck is very smooth and quiet. You can sort of replicate the sound by tapping on the driveshaft. Can you tell if the shaft is out of phase by looking at the U joint coupling?

Thanks,

DT10

(Edited by DT10 at 3:32 pm on Sep. 6, 2001)

[Shaners]

I would have thought the the person that did the gear swap for you would have marked the shaft with chalk or something when he removed it, so that it would have been replaced in the same spot.

What year is your truck DT, and how many mile are on it?  And how many miles since the gear change?

[BIG BLUE]

Yes you can tell if the shaft is out of phase.  Look to see if the slip yokes are parallel or lined up the same from the front to the rear looking down the shaft they should match up.  However, if a shaft is out of phase then the results are usually a vibration more than a "whir" in my experience.  

Is it possible that since it's a 2wd, that your wheel bearings are the culprit?  They will sometimes make the sound you describe but not over just certain ranges usually.  

BIG BLUE

[DT10]

Thanks for the replies. My truck is a 2001 with 13500 miles (I do a lot of interstate driving).  The gears now have about 700 miles on them and sound has not changed. Like I stated before, the thing is whisper quiet at 85 mph and very smooth. I think it may just be a harmonic since the rpm of the drive shaft is a bit higher relative to everything else.

Thanks,

DT10

[Shaners]

BY out of phase I understood it to be did he put the driveshaft back in the same spot...  am I mistaken on this???

The other thing I tjust thought of would be is the pinion angle set right??? Did he remove the whole rearend or replace the gears while the rearend was in the truck????

It does seem as though it is harmonic rather than mechanical because the pitch never changes but rather it re-occurs at different high-speeds/revolutions.

Since it is a 2001 you might be able to slip it past a dealer for a warranty item as long as he doesn't know you changed the gears.

BTW, did you reprogram the computer to accept the new ratio?  If not then your speedo is way off...

[DT10]

Shaners,

The gear set was replaced without removal of the rear axle assembly. Yes, I did reprogram the ECM via the Hypertech PP3 I use. The speedo/odo are right on according to my GPS. The Tow/Haul shift points are about 5 mph lower since they are RPM referenced but the Hypertech unit moved the standard shifting up in synch with the gear change.  I have a limited warranty on the job so I will continue to listen over the next few months. This guy builds a lot of differentials for street and track cars as well as heavy towing rigs. I will keep you posted. Thanks for the ideas.

DT10

[Stepside Z]

DT10,

First welcome to the site. Second it sound like the gears are not meshing correctly. I had a 88 chevy that did that and the dealer had to replace the rear end.