Any ideas as to the cause of this noise?

[veteran]

For at least a year now, I've been trying to pinpoint the source of an annoying metal-on-metal 'clank' noise on my Polo 6C. I'm hoping that if I describe it and the circumstances under which it happens, someone in these forums might be able to fathom what it is, or maybe have even encountered it themselves.

The noise is momentary and occurs whenever either the clutch pedal or brake pedal is released. It's percussive in nature and comes from somewhere around the back half of the engine compartment. It's loud enough to be heard within the cabin, even with the engine running and the car moving.

Whenever the clutch pedal is operated the clutch's last stretch of solid pipework before the slave cylinder, as seen in the engine compartment, flexes visibly - which clearly shouldn't be happening. With someone sitting in the driver's seat and operating the pedals, I discovered the latter while myself listening in the engine compartment. A thorough examination of the car has shown there to be no leakages of brake/clutch fluid. No vehicle faults are found on the driver's display. The brakes seem to work fine, and gearchanging is without issue too.

I've recently conducted a check of the brakes themselves and have also completely changed all the brake/clutch fluid and that's not made the slightest bit of difference to the noise. The clutch and brakes do, of course, use a common fluid.

I find that, where the brakes are concerned, if I only lightly brake when I'm out and about I don't get the noise, but any reasonable force applied and there it is. As for the clutch, it happens every time the pedal's operated, no matter how far I depress the pedal, beyond a certain point.

In a motoring magazine I've seen an electrical circuit diagram for my 1.2 TSI that suggests that the vehicle makes use of a 'clutch position sensor' and a separate 'brake position sensor', with outputs going to the ECM. Assuming that these do exist on my 1.2, could it be that these are, in effect, heavy-duty spring-loaded switches and so may be responsible for this noise? I'm fairly certain a clutch position sensor of some kind exists for instance, because you can't start the car without the clutch pedal being in the fully depressed position. Presumably, if both such sensors are fitted on my Polo, they'd be located somewhere around the RHS of the bulkhead/firewall?