If the car does not have a tandem pump leak there is a good chance the foam has never been removed, the diesel destroys it. Only reason to remove it would be to work on the head area say injector loom change. The foam does deteriorate over time and some may well have crumbled away by now.
From my own experience, if the foam is missing and only the fibre cover is present I would say the car is about 20% noisier than with both foam and fibre fitted. If none are fitted then the noise increases by about 20% again. Not exact figures just my thoughts. My foam lasted intact until last year when a tandem pump was firstly resealed and then replaced... along with all the rubber hoses twice.