My car started making an awful metallic rattle from the auxiliary belt area last night, accompanied by the battery light flashing.

Today I took the belt off and checked the main suspect, the alternator free-wheel pulley, which seems to be fine.

The tensioner and all the other pulleys seem okay too.

What else could it be?

If the battery light was coming on, then that suggests the battery isn't charging so either:

- belt

- tensioner

- alternator pulley

- aircon pump pulley

When my tensioner failed it made a "groan"y noise which got louder before it eventually broke. The metallic rattle sounds more like the tensioner unit, imho.

Thanks for your quick and helpful reply.

The belt looks nearly new.

The tensioner was chattering a bit last night.

Alternator pulley tested (screwdriver wedged in the vanes, pulley goes one way (clockwise IIRC!), doesn't go the other way, no play)

How do you test the aircon pump pulley?

It has 171000 miles by the way. Drives really well, but I guess a few things might be borderline.

The pulley on my car alternator broke off at a similar distance.

Is the belt nicely tensioned?

Not really sure how to check the a/c pulley - it should be smooth to rotate - it has an internal magnetic clutch to engage drive to the pump when a/c is required.

I put it back together and it seems to have fixed itself, no more battery light or noises. Belt's nicely tensioned.

Pretty sure it's the tensioner, I'll source a new one ASAP.

Is it OK to just replace the tensioner strut? http://www.ebay.co.u...10-/14084032256

I would bet the pulley on the alternator is bust and has killed the tensioner. I tried changing the tensioner but it started rattling again and so I changed the pulley at that sorted it.

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Just to clarify, the alternator pulley isn't showing any of the signs of being bust.

Maybe the alternator pulley's been replaced before and the tensioner's worn out from past abuse?

My seemed OK in the car until I fitted the new one and then you could tell the difference. With it in the car you can really only check it isn't seized, not if the bearings are a bit rough.

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Thanks for the advice, I'll replace both the alternator pulley and tensioner.

Thanks for the advice, I'll replace both the alternator pulley and tensioner.

you need a special tool to change the pulley and the alternator has to come out the car.

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for the cost of the tool required to change the pulley on the alternator it is worth seeing if your local friendly garage will change it for £10. That is what I did, but it only cost me the change I had in my pocket (iirc about £1.50 )