Hi,

the records of the radio messages on Flight 93 can be listen here.
There is a strange background noise at the radio messages(pilot and hijacker) of Flight 93.
Here is a Webside with Cockpit-Alerts. That one for the firebell of a Boeing 757 is called "Firebell (modern)".

Other Cockpit-Alertsounds of a Boeing 757 you can download here.
That one for the firebell should be the b62s.wav and can be directly downloaded here.

Is it probable that the strange background noise is an firebell-alert in the cockpit?

No, it is not.

If it were a firebell warning, the warning would have had the full attention of the crew and they would not be making routine radio transmissions.

The first response to a firebell is to silence the bell,so the crew can then apply the proper procedure and run the appropriate checklist. It is not a warning a pilot would ever ignore. Once the firebell sounds, the emergency takes precedence over all other actions, other than maintaining aircraft control. Any transmission to ATC after a firebell sounds in the cockpit would likely include the declaration of an inflight emergency.

To me, it sounds like an electronic anomaly associated with the transmission.
Regardless, since it was unique to United 93, it deserves further consideration.

When the firebell warning began, the hijacking probably already has taken place. Perhaps there would be no chance for routine actions of the pilot and the crew.

What could have caused an electronic anomaly? Do you know a similar example in radio transmissions?