Cisco LAN :: 1801 Unknown Protocol Drops On Connected Router Interface

I have a 1801 router connected to a 3550 switch with a regular 802.1q trunk, and I am curious as to what may be causing the unknown protocol drops on the connected router interface.
 
The switch is without any configuration at all except the following for the trunk configuration on the interface connecting to the router.
 
Switch:
-interface FastEthernet0/1
-switchport trunk encapsulation dot1q
-switchport mode trunk
 
Router:
-Interface FastEthernet8
-switchport mode trunk
 
There is nothing connected to the switch other than the router so the dropped traffic must be originating from the switch itself.The unknown protocol drop counter on the router increments by one every 30 seconds, and I tried using a packet sniffer but nothing noticeble showed up.
 
I read elsewhere on these forums that it might be udld, but that is not enabled by default, and just to be sure I tried disabling it on the interface and as expected it said it was not enabled, so I am ruling that one out.I also read that it could be because the router is recieving traffic from other protocols than IP, but I do not see how it applies in this case.

what does a 3550 send every 30 seconds that my 1801 does not understand?Could it have something to do with STP?