Popping / Knocking Noise After Service

[DaveM]

Had my '98 2.8V6 Galaxy serviced last Thursday.

 

Since the service i've noticed a knocking/popping noise coming from under the air filter box, (top left hand side under the bonnet)

 

it's an intermitant rythmic knocking noise, 2 or 3 knocks a second that only happens when the engine has warmed up.

 

it will suddenly stop, but then start again a few seconds later.

 

you can only hear it whilst idling, but then the engine noise may be masking the noise.

 

To my un-trained ear, it sounds a bit like a air non-return flap installed the wrong way round, so the air is banging against the closed flap. The intermittant nature suggests it's thermostatically controlled.

 

Is there such a thing behind/underneath the air filter box ? maybe some sort of idling valve ? or am i just guessing and getting it completely wrong ?

 

The garage ensures me there is nothing to worry about and it's probably "one of the two rubber pipes at the back of the box that are worn and need replacing"

 

They are going to take it back on Thursday, but for my own piece of mind, i'd like to have a few ideas for the cause of the noise.

 

cheers