I left it somewhat vague as to where Willem was. So, it might be plausible if Karl and Jessamy somehow got turned about a bit, that they might hear his van. I think, besides the crunching of the leaves and the horsey snorts, they might also hear the clink-clanking of various tools, pots to be mended, chains, whatever, that are probably hanging on nails just under the van's eaves.
Could sound like some old wight or other creature rattling chains or sword as he clomps along in search of a victim.
That could set the girl off screaming and possibly raise the hair on Karl's neck.....
