One firewall in a VPN pair pinging the other?

I have a pix 506E on one end of an ipsec site to site vpn that seems to be
pinging it's remote peer at a constant rate. Almost every second. This
traffic wasn't logged before I started at this job and not until a week ago,
so it's difficult to tell if it was happening prior to the VPN being created
(which was a couple of months ago) or not. Can anyone tell me if this is a
standard function, or if something is amiss? Thanks for your help...

Dan Denton