Diagnosing rattling noise whilst car under acceleration

I have a 2008MY Fiat Grande Punto (6 speed manual transmission), which has developed what I would describe as a rattling noise during acceleration.

A few observations I have made:

• The noise sounds like it is coming from underneath the car, at the front, approximately where the drivers feet are positioned by the pedals.

• It occurs only when accelerating. No noise can be heard when stationary and idling, nor when driving without pressing the accelerator…

• … however, if I rev the engine with the clutch depressed, no noise can be heard as the revs increase, but a brief noise can be heard as they begin to drop.

• No noticable loss in power, or degredation of fuel economy.

Checks already made:

• The exhaust system (middle and rear sections) were replaced last year but I had the exhaust shop take a look yesterday and they ruled out an exhaust problem or a heatshield loose

• Shop has inspected the suspension/wheels and found no problem there

• No engine light / emissions fault codes, with several hundred miles clocked since the problem began occuring

To me it sounds like the same noise as when my flexi pipe broke, but the exhaust shop informs me it is fine.