I've recently disassembled a B18 so I know the innards, but I'm not much of a diagnostician...
The fibre timing gear would be the first thing to check. I hear that they can come apart but may have enough intact to keep the engine going. The broken off bits will "slap" against the timing cover. Another possibility (that could cause a slap noise) might include the lever arm on the fuel pump (which is operated by the cam). I'd think that anything involving the pushrods, cam followers, or rocker arm setup would make a more localized noise and probably not a slapping sound.
A problem regarding valve #4... Not sure there, but it might indicate some kind of head trouble. Maybe a bad valve spring? I'm not to knowledgable about head troubles, but those spings acting on a bad valve might aslo make a slap type of noise.
Good luck.
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1967 P220 Amazon, 1972 145S, 1976 245 DL, 1983 245 DL, 1986 745 GLE, 1990 745 GL, 1995 945.... You mean to tell me that Volvo makes cars that are *NOT* Wagons?!? 1971 P1800E... Not a wagon, but it's just a donor car for the Amazon...
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