2010 Aygo - metal scratching / grounding noise

[Tomos88]

Hi

Over the last 3 months, intermittently i get a scratching / grinding noise from under the car.  The only way i can describe it is, it sounds like the exhaust is dragging on the floor.

It only happens maybe once a week, and normally disappears after a few minutes especially when going above 20mph / 3rd gear.  

It happens both driving straight and at corners.  Until today, its always happened with just me in the car so no additional weight in the back.

I took it up my local garage a fortnight ago.  They took it for a test spin, and did inspect the car, but they never heard the noise nor could find anything.

Today it happened again for, this time while i had a two passengers in the car, and it happened for a full 5-10 minute journey home.  They described it as sounding like the exhaust was on the floor.  We were mainly going sub-20mph, and went from 1st to 3rd gear.  it continued making the noise to while in neutral (coasting down a hill).

The garage did say, as i noted it sounded like it was from the middle / rear of car, it could be something within the back brakes but they would need to strip them down so we parked that idea at the time.  They were comfortable though it was not just worn bearings.

Has anyone else ever experienced similar?