Hello there! I'm Eurocar ! I will try to assist you as much as I can. Quality research = Quality answer!!
that air compressor is for the air suspension for the vehicle. the suspension is auto leveling. it is not like the older RR and Bentley setups because they were hydraulic.
some noise from that is normal.
also there is on some models an brake vacuum pump that is electric and with some W12 engines the vacuum is not enough to operate the power booster for the brakes so an vacuum pump is necessary for certain conditions.
some noise will be normal . they do come apart on the inside and make more noise than usual then they are deemed to be replaced.
since you have replaced the pump the noise is more subtle now but some noise will be there .
also with the compressor pump for the air suspension will make some noise too to keep pressure to the storage tank (over 100 psi) .
I understand that Bentley is a fine automobile and you just purchasing it you expect high expectations too. I do know safety is a big thing for them and for the customer.
with out hearing these noise is hard because .... you know but since you have replaced the brake vac pump already and you noticed a difference in noise level but you do still hear the noise. some noise is normal . if it is a more continuous noise for a longer period of time I would investigate it more than short bursts.
I hope that helps out !
please let me know if anything else
Thank you for answering. My mechanic also told me he detects this sound on every Bentley he encountered, and noise in my car is in low level (after replace the pump). Just I am not sure how can a fine car like this the maker does not try to improve this noise??
Will this pump heating up during working condition? In your opinion, is it possible I can do some isolation try to reduce the noise down even further?
I think Bentley should move this pump to rear end of the vehicle or do some sound insolation on it.
Anyway, thanks for confirming this for me.