1996 Laguna 2.2 Diesel, strange noise?!

[danny_blair]

Hello,
I've just recently purchased my Laguna and in general, I am very happy with it. The car has a good ride, comfortable to drive and good on diesel economy - especially compared to my last vehicle, a 1985 VW Camper!
I have some concerns about a noise I am hearing, and wonder if anyone has any experience with this. At low RPM - between about 1100 and 1400, there is a rumbly rattle. At idle (800) this is not present and while driving I am pretty sure it's not there. Just when the car is idling a bit higher than usual, or when starting off in 1st / 2nd gears I can hear this noise. It is reasonably loud.
I've had a look under the bonnet and even taken off the cambelt cover to make sure everything was tight. I can't see or hear exactly where it is coming from. Yesterday I took it past a mechanic who used some stethoscopes (?) on it to try and work out what it was but couldn't tie it down. He suggested bringing it in and they would take all the covers off and get it on the lifter. I'd rather have a bit of an idea before dropping it to someone who is just going to be guessing! I could see it getting expensive...
Has anyone seen anything similar to this, or have any ideas what the problem could be??
Look forward to your replies!
Cheers,
Danny