Suction when taking off the fuel cap means the fuel tank vent pipe is blocked.

if it is vacuum sound not pressure sound you need to examine the EVAP control system and valve ( EVAP system stops petrol fumes from fuel tank venting straight to outside air) usually this system is under bonnet , don't know for sure where on your car.
remember that sloshing fuel in tank will build vapour pressure in tank in short trip / time.
If no air reaching tank and vacuum forms it will not cause pump to fail but may cause tank to collapse.
Try not to over think it , it may be fine.

Thanks for the replies.
I ll be looking at the petrol vapour filter (the part will be easy to get but cant find any info on how to replace that, or what needs to come off to gain access to it).
But that will be a job for another day.
I ve made a cap with a breathing valve (same i used to have on my bike). it will do for now...

It's been a few months but here s a quick update on the vacuum effect in my petrol tank.

I did a few miles (30) with a band new OEM fuel cap on.
Stopped, opened the cap: big "swoooooshh" noise.
Another 30 miles and same thing.

I did an ECU reset and went on my way, not expecting much...
Success, no more vacuum since...