Trans leaking from top vent at rear

3L engine, 3 speed plus OD trans. No unusual noise or debris in the pan. Took a long time to figure out where it was leaking, but fluid is coming out of the little top vent at the back of the tranny. All functions of the trans are good. Just replaced the filter and modulator. No leak while at idle whether in gear or not, only after driving. 5-10 miles. Fluid level is correct (until it leaks out) I would call it a fast drip and leaves a palm sized puddle after the first stop when full, and puddle size diminishes there after. OD never worked since we got it 40k ago. Leak is recent and seems to come and go, but right now is steady. We are good mechanics but need info. We appreciate your help.

Glenn