Pounding Sound?

[SMH1982]

This seemed to always bugged me, and maybe not everyone will agree, but....

right after the part where THX and LUH are praying/worshipping together, they begin to talk. During this time there is a somewhat steady beating sound, or pounding. I was wondering what the hell is it? Is it suppose to represent the heart beat, or is it the sound of a Chrome Robot beating down some neighbor?

Any ideas?

[IJT-7992]

I always assumed it was one of the neighbours watching the 'chrome-bot' channel. Could be wrong, though...