Mystery noise: engine, gearbox?

[Mew]

Unfortunately the diff was one of my original suspicions, but i didn't want to put it up here incase someone just agreed. I wanted to see if someone said it without me saying it. I haven't looked into it any further though.

I also discovered yesterday that it doesn't do it in 4th gear, only 5th, so an oil change is probably a good idea, even if only to ensure the box has the correct amount in there.

As for the release bearing, there's no noise at all at idle.

The 9a is an 020 box btw, same ratios as the 2y except for a slightly taller 5th

[kraM]

pray for no twinkley bits of metal in the oil then...  Dunno if you are aware, but the 020 box evolved slightly over time, read somewhere that the box was orignally designed to be used level, but it leans back by 30' in the golf.  The result is a lot of cars get run with too little oil in them, they should be filled to the bottom of the filler hole, then put the bung in, then take out the speedo drive and pour in a futher 500ml.  I know you said yours is a 9A box, so this is probably irevelent, but maybe not.

Also whilst you're down there, grab a hold of your flange and give it a damn good wiggle, there should be very little play in the diff bearings, or...

Jack it up, both front wheels free, make sure it is as secure as possible, lots of axel stands, chocks etc, fire her up in 5th gear.
Gingerly get under the car with either a stehoscope or long screwdriver and have a listen to the diff bearings, then with a gloved hand, providing no LSD, slow 1 of the wheels to a stop (be very fu****g careful, that put extra load on the diff bearings and they should reveal their wear to you, then repeat on the other side)

Also the diffs are held together with rivots, which are renowned for being poo, bolts are muche better.

kraM

(bringing you happy thoughts of taking gearbox's out.)

[Mew]

I also discovered yesterday that it doesn't do it in 4th gear, only 5th

That was a lie. It does do it in 4th.

I've had a thought, although i'm not sure it explains the lack of noise when not under load:

What's the verdict on a dodgy PS pump causing the noise?

[Thom89]

Try driving the car with the PAS belt taken off...
Did you try the car with the wheels swapped front to rear?
Thom

[Mew]

Can't say i have. I'm struggling to see the logic though?

[Thom89]

Any uneven tyre wear, flat spots, damaged side walls, even the thrown out grease from a split CV boot,  will at certain speeds set up a drumming noise that resonates through the car, as the front suspension has more linkages and joints etc, then the sound gets amplified,
swapping front to rear, will tell you straight away, whether its the tyres or not, rather than guessing at everything else..
Thom

[Mew]

Well i'll give it a go on Wednesday, but i would have thought that if a tyre were to blame then the noise would be there regardless of the engine being at 3.5k or 1k at 70mph

[Thom89]

Your O/P describes perfectly the symptoms of uneven tyre wear, right down to the noise changing / disappearing under load, and coasting...
Thom

[Mew]

Oh.

When i got my first gti i thought it had a dodgy wheel bearing, but that turned out to be the tyres (kwikfit own-brand, didn't even know they did them!). That was a constant drone though rather than more of an oscillating noise like this.

Anyway, i'll swap them over on wednesday and see what happens.

[Thom89]

At least you'll know either way....
Thom