Transmission cooler. If you remove it, replace the o-ring and be sure to recheck the ATF level when you finish. Maybe even service the trans if it is due.
Personally, I think this level of disassembly creates more issues than it solves. I will wait for your "no AC" post in the spring.
Growing old is mandatory; growing up is optional.
2005 A6 Sedan Q 3.2 (wife's whip)
2003 S8 (Q Ship)
2000 A4 Avant Q 2.8 (daily driver)
1999 BMW M Coupe (clownshoe)
1980 VW Scirocco (mid-life crisis car)
1983 Mazda RX-7 (revolutionary sports car)
1999 Mazda Miata SSB (track beotch, no plate)
2003 GMC Yukon Denali XL (the anti-earth)
2004 Dodge Dakota (hey, at least it's stick and 4x4)