I've only heard Sancho's rumbles and flatulence if I have my ear (or hand!) close to him. I'd never heard this before I got Sancho, neither of my girl guineas had ever made these noises. When I heard it, I was worried about bloat immediately and reduced all gassy veg. but 5 months on (he's a 2 year old rescue pig), he seems perfectly healthy and is on the same diet as my other pig. His stools are normal and his stomach is firm but not distended or hollow sounding.
I thought it was safe (in this instance) to conclude that it is just the noise his tummy makes, but I would be interested to hear about cases where these noises may indicate more serious problems. I know I should look out for signs of blockage and bloating but I thought serious bloat (which causes major discomfort to a guinea) would be coupled with other symptoms; lack of appetite, lethargy and a change in bowel movements.