Grinding noise after 57 Passat B6 TDi 140 has been standing for a while

[Pharlapdog]

This one has got me foxed.
After my 57 Passat B6 TDi 140 (160K miles) has been standing for a while and I drive off I get strange grinding type noise when I apply the brakes and vibration I can feel through the brake pedal.
This only happens at low speed. If I don't apply the brake and get up to some speed before braking, there is no noise/vibration. After a short time the car drives normally until left to stand again.
I did have a problem with the offside front caliper sticking but that has been replaced and the problem seems to be getting worse.
I have jacked up the front of the car, put it in first gear and allowed both front wheels to rotate. When I apply the brakes, the noise seems to be worse on the nearside front, rhythmically scraping several times with each rotation of the wheel (not just once per rev).
Then I tried jacking up just the nearside of the car and following the same procedure. With the offside front wheel on the ground, the nearside one rotates twice as fast. The rhythmic scraping is still there but faster of course.
Then I tried jacking up just the offside of the car and following the same procedure. With the nearside front wheel on the ground, the offside one rotates twice as fast. The rhythmic scraping is gone but there is a different kind of noise under braking which seems to be coming from the engine rather than the wheels.

My initial thought was nearside brake caliper binding like the offside one did, but if it was that, why would the fault disappear at high speed?
Then I thought it might be the nearside CV joint. Both boots seem in good condition though and whilst it could be that, why do I get noise from the engine under braking when that wheel (and driveshaft) is on the ground and the offside wheel is free to rotate?
Is it possible that the noise is associated with the differential and there is a gearbox fault?

Any advice gratefully received.

Pharlapdog.

[Rob69]

You don't say how worn the pads and discs are but I think for a start you should strip down the front brakes and check discs for run out on both sides, check for rusty splashplates catching on the back of the discs, check the pads can move easily in the pad carrier brackets and calipers can slide on the pins ok.
I'd be very careful doing your jacked up running in gear tests, there is a risk you'll pull the car off the jack ( and hopefully stands) when you're applying the brake, as you're transferring torque into the static braked wheel on the ground, that is probably the noise you're hearing from the diff area as the spider gears in the diff are rotating far faster than they would do in normal driving and are being loaded up as you apply the brake to the rotating wheel, your abs system will probably be getting confused as to whats going on as well, maybe the noise is the ABS pump kicking in.

[Pharlapdog]

Thanks for the advice Rob69,
Pads and discs were replaced in December 2017 and are in good condition.
The ABS certainly did get confused and generated a fault code which I had to clear with my ODB reader.
I'll check the nearside pads & caliper but I'm not hopeful as it does not realy fit the facts.

[Crasher]

I would suspect you have one or more corroded rear wheel bearing magnetic rings.

[niall campbell]

Could it be that the pads have been put in wrong ? One goes in the piston and there might be slight movement at low speed, therefore the noise. Higher speed and its under more pressure so will stay rigid.

I would also take off the disc and wire brush area it touches the hub carrier. It could be rust in there that is rubbing against the disc, could be the disc welded itself to the hub causing a slight rounded lip. You then moved the car breaking this rounded lip and every time it turns and hits each other you get a noise

Then again maybe the rubbish in the inner wing dried out and fell into the brake assembly

You need to take off pads & discs and clean everything, use some copper grease and check it out ……………………...

[Pharlapdog]

Well, I replaced the front nearside caliper today to no avail. The grinding noise is still present.
An other thought occurs: Suppose a seal has gone in the gearbox and it is almost dry? Higher velocities will fling any oil left around the gears and make the noise disappear?

[Rob69]

If you'd lost that much gear oil to a seal leak it woul be pretty obvious, be worth checking the level though. A good point raised by Crasher about the ABS rings. These do corrode and build up scale which might be grinding away. Ive seen them grind the tip off abs sensors, but that flags an ABS fault code, and the rear ones push out the plastic cover ring from the back of the hubs.

[Pharlapdog]

A good point raised by Crasher about the ABS rings. These do corrode and build up scale which might be grinding away. Ive seen them grind the tip off abs sensors, but that flags an ABS fault code, and the rear ones push out the plastic cover ring from the back of the hubs.

Spot on! That's exactly what the problem was. Many thanks Crasher et al.