On 145 when you taxi out on the number 2, the pumps make a bit of noise in the cabin....well I can't even begin to tell you how many times I've been in the back deadheading, and a pax will turn around and ask me "if I'm going to do anything about it", "the plane is broken", so I just smile and tell them is normal. Any other time if I'm flying, I acknowledge their concern and explain that it is perfectly normal (whatever the "problem" might be).