(G) Noisy Fuel Pump

To cut a long story short, I replaced my in-tank unit with a new Bosch unit. I find that when I start the engine up you can hear the electric motor working all the time, it's loud, and you can here it fluctuate up and down with voltage changes, like if you turn the lights on when the engine is in tick-over. Although the unit was new, it was out of warranty, as I had had it laying around for some time before fitting. I decided to change just the pump, as I had a spare Bosch pump only from my old unit, which I know, did not make any noise. However on taking the pump out of this new unit, found it was fitted with a Walbro GSS342 pump! OK, if that was was fitted from the factory, then fair enough! But this is a 255lph pump and the pump that was fitted to the car from the factory was 90lph!

However, there are differences in the motor body length, so I could not fit the Bosch pump in this unit. Luckily enough, I had a new uprated Walbro 255lph Fuel Pump from FSE Sytec (ITP116), which to my surprise, produces the same 255lph as the factory stock GSS342 as fitted to this Bosch unit! Anyway, fitted this new pump and this noise is still there!! Any ideas?