Transmission leak from unidentified plug?

[NorthCountry]

I have a 2002 GMC Sierra 2500HD Duramax with the Allison transmission. 95,000 miles. Bought it used three months ago. A few days ago I spotted a small spot of transmission fluid on the ground under the truck. After further investigation I find that only when engine is running there is tranny fluid seeping from some type of aluminum plug or vent-like apparatus, but it is on the bottom of the housing. This thingamajig is just forward of the transmission pan, and left of the spin-on filter. It appears like aluminum, or at least, is shinier than anything around it. It is round and convex-shaped, and has 4 ears at 12, 3, 6, and 9 o'clock positions. It is a little bigger than a quarter.

 

I'm sure a seasoned Allison person can tell me what this thing is, why it is leaking, and if it might mean I'll need to take out a second mortgage?

 

Trans. fluid level looks ok. Dripping once every 4 seconds or so. No noticeable change in performance or shifting. Trans. temp gauge reads normal.

 

Any ideas? Thanks in advance!