Hi,

I run a 1988 Firebird 350 Sport Coupe, whose drive train is TH700 with overdrive and TCL, and a Borg Warner PN1005423 Posi (Final ratio 3.27:1).

Just lately, there's a strange noise from the back end when the car is manoeuvering at low speed, e.g. in car parks - it's a kind of mechanical whine, quite noticeable, whose pitch increases with the tightness of the turn being made - so it starts lower in pitch when the steering is first turned, increases in pitch as the steering wheel is turned further and the turn radius decreases, and then decreases in pitch as the car straightens up again. It almost sounds like an electric motor running up to speed and slowing down again, but the tone is (almost) like rubber against metal - can't believe that is what it is, though.

Is this coming from the posi / diff, do you think? All opinions valued!

Thanks!

 

Could be. Have you drained the gear oil lately?

GD

 

Do you perhaps own an FSM? If so, there is a very good write up on what noises pertain to when diagnosing the rear end. It is section four. The situation you describe could be rear axle side gear and pinion noise or pinion bearing noise. If you are interested and want to read more, it is on page 4B-3.

 

Not recently - the car has covered very little mileage in the last four years (due to a water leak that's now fixed at last) and it doesn't drip oil from anywhere including the rear axle, so I have not checked on it recently. The noise has not been apparent before the last two short journeys - so it began somewhere in the last 30 - 40 miles. It's only noticeable at low speed, and most of my driving is on faster roads, so I can't say exactly at what stage in those last 40 miles it began, but certainly it was not there more than about 40 miles ago.

 

I do have the service manual - had not got around to looking in there yet. It is amazingly comprehensive compared to other car manuals over here in the UK - can be hard to find the right info in all that mass of data!

Thanks for that suggestion. I'll look in there now!

 

The cone type posi is prone to wearing out and once that happens then the side gears could be making noises, etc when turning. Not to mention the cones, etc.

Highly recommend swapping the cone type posi for a Torsen. We have a few write ups in this section on it. Since you have 3 series gears you can just swap it in without changing gears. All you need is carrier bearings, shim set, and the torsen carrier from Australia.

GD

 

The first thing that should be done is to change the oil. This is the easiest and cheapest thing to do and most of the time it fixes this type of noise. I recommend using the GM limited slip oil additive and give it about a week to start working. If this doesn't fix it then you may need to go inside the rear end and check bearings and other parts. If the cones in the Borg Warner unit are worn out then you need to change it or recondition it. The Torsen is a very good idea too.

 

Could an axle bearing cause this noise?

 

Axle bearing would be creaking/growling at low speed and rhythmic humming/howling at higher speed and would not stop when going straight. If it's only in slow turns it is almost certainly carrier related.

GD