Mine was quite infrequent, but as I had the warranty I decided to get it checked out anyway. It was a bit difficult to get the car to actually make the noise while the mechanic was present, in fact it was on 3 seperate visits before it would actually do it.
Then I had to try to get it to repeat the noise the following day for the warranty guy. Nearly half an hour of trying while he was standing outside with the mechanic from the dealership when in suddenly let out a loud scream.
I was amazed that the warranty people agreed to pay when it clearly was such an intermittent fault, but they did.