Off the top of my head, I think my Humber is between 2 and 3 psi for tube pressure. I bought a little guage with the rib so I could check the pressures - I know they're stated on the console so that's what I generally go by.
If they're soft, I also get that noise I think which you refer to as a 'drumming' sound at the stern end when at speed especially when starting off first thing in the morning.... once the sun is out that goes completely.
I went out on a rib which had tubes without a great deal of air in them a couple of years ago, and it heeled nicely to port
- later on that day when we found the pump and the air was put back in, the thing was spot on level again.