What sound does oyster card make if it is not in credit?

[filthiestfis]

Yesterday I was on a bus when 5 youths entered (through the front door as opposed to the usual back door shenanigans) and when all 5 touched their oyster cards on the reader, it made a BEEP BEEP BEEP noise, unlike the BEEP that it makes when your card is in credit. The first thing I thought was that the driver has let them through in the knowledge that they dont have credit, however, could it be that there is a weekly or monthly oyster top up that makes this noise? To me it made the same beeps when I touch my oyster card that has no credit.

[SlackAlice]

A single BEEP denotes the Oystercard has been accepted as valid for travel. It does this for all types of passes ie PAYG, Monthly , Weekly, Daily and either Bus Pass or Travelcard
A double BEEP denotes the Oystercard has not been accepted as valid, or the Oystercard Reader has failed to read the card properly. The driver gets a seperate display to say what is wrong and either tells the passenger to try again or tells them the Oystercard has expired or run out of credit.
The only exception to this is the Child Oystercard which gives a rapid series of BEEPS (7 in all) when it is accepted by the Oystercard Reader.