My 72 Fastback has also had this sound deadening felt (A friends 72 Humber Sceptre also had it) and on mine it has also fallen/been pulled off (before I got it) leaving wispy bits of fluff. Had some success with this stuff.
My 72 Fastback had foam and fabric underside of the bonnet. Unsure if this was factory installed. However, it all fell off so having scraped off the foam I now am unable to clean the "glue" residue off the bonnet. Can anybody suggest a solvent to clean the steel?