My TD does that periodically. It's done it maybe 4 times in my 8(?) years of ownership. I know it's about to happen when the downshifts get harsh. Have no idea what it is. Everytime it has happened, problem has disappeared the next time I start it. I figure maybe a sticky check valve in the pump? I say don't worry about it until it happens again and doesn't go away.

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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