Unknown UPnP Port

I have 2 PC's setup with windows 7 x64 7600, one is my main PC and the other is my HTPC.

My HTPC is opening up UDP port 54838 via UPnP on startup, with out any applications starting.

This is a fresh install with no software other than iMon which is for the remote and VFD.

I have run tcpview and currport to see what is doing this, but they only show that the port is open and not what is associated with it. Have also run Sysinternals Autoruns, which lists everything that is running as a service or as a program, and I can see nothing suspicious !

I also have downloaded a free version of UPnP Explorer, but this only displays info on the physical router side of things.

I have searched the net and can find no mention of UDP port 54838 anywhere whatsoever, and have checked all of the port listing sites and none of them have any details of this port number, other than it is in the dynamic private range ... so it could be anything

I have tried to create a firewall rule to block it, but UPnP just creates another rule to enable it !

I know that the simple answer is to turn off UPnP, but I do actually use it for quite a few applications and devices so it would be a pain to do so as UPnP works really well with multiple devices that switch from wired to wireless mode and with dynamic IP's.

Has anyone got any ideas on a) what this port might be and b) if not, what else I can do to track it down ?

I don't want to get paranoid about it but I am security conscious enough to want to know what is making connections outside of my network with out my permission.