MK3 Galaxy - 2.0 TDCi - 2007

Have tried to identify the source of this noise for a while now but have been unsuccessful. Dealer also unable to replicate fault.

Approx 2-4mins after switching engine off a noise coming from the engine bay can be heard which last for about 5secs. It starts at its loudest and then fades away to nothing in the last couple of seconds. Best way to describe it would be a deep sounding tuning fork which is struck and shortly after fades away.

I've tried to find scenarios where I can confidently get this sound to occur but I just can't find any. Sometimes it does, sometimes it doesn't !!

However at the weekend I had a new scenario where this fault was occurring. About halfway through the Blackpool Illuminations at the weekend it started happening again. Occurring most times when at idle I could feel a great deal of vibration through the pedals and floor pan in addition to the noise.

I suspect it could be the electro-power steering motor but would appreciate the experience of others if this has happened to you.

Thanks.