Latitude E7440 speaker clicking

Hello,

I've got a E7440 running WIn 7 SP1.  Every so often when I change applications, there is a loud single static-type click from the speakers.  I've de- and re-nstalled the speaker drivers in Device Manager but it still happens.

It's not the end of the World, but it's annoying.  Would be grateful for any ideas on fixing.

Thanks,

Gents99