there are 2 hoses that go from the back of the engine to the firewall in that area. they are prone to getting small holes that only leak when the pressure is right. Take them both off and inspect them. Bet you'll find cracks if you can get a good look at them. my 90 lwb had this same problem.

'88 Raider. 2.6L dead...2.4L swap in progress, AT, 33's, 2 bouncy seat,AC, CC, 2.25 exh, 2" Tbar, 2" BL, rear DB, rear LSD,4.90's, SW hubs,gen2 uca's, KYB shocks, snorkel, elec fan, MSD.

'99 chev k3500 crewcab
'15 Subaru XV
'03 BMW i325