It's the opposite of a leak. The ignition switch, when turned to 'go' is supposed to bleed off the vacuum that was introduced to shut off the engine. If it doesn't the shut off diaphragm is still engaged. This problem usually manifests itself during short stops when the vacuum hasn't slowly bled off over time without the ignition switch. Hence the significance of 'it only happens when I go to the bank' thingy.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13