I think it’s OK if you are looking to buy a s/h car but a nuisance if you are trying to sell one.
My wife’s car has one or two, one says there is a knocking noise coming from under the car but no-one can identify it even on a road test. I’ve never heard it and nor has she. Could be the garage is touting for more expensive imaginary work to be done ■■
Intermittent faults do occur but unless it was a garage I knew and trusted I’d be sceptical 
just turn the radio up a bit an it i’ll be fine 