random beeps during backup

[griffin tucker]

my debian testing virtual machine randomly beeps, sometimes really
long beeps, when running rdiff-backup after a long time (overnight)
and only when interacting with the installation while it's still
running

nothing in /var/log/messages

rdiff-backup doesn't report any errors (empty log file)

i've limited it's processor count to 2 and re-run it, and there's no
beeping anymore

i'm just wondering, can i cap rdiff-backup's cpu usage to a percentage
so i can utilise all cores but not experience system instability?

otherwise, can i cap rdiff-backup's thread count?