Originally Posted by
granto
(because I know nothing about freewheels), why would it only happen when I'm in the highest front gear?
Sorry I missed that detail in the OP. It could also be, as FBinNY says, an issue with derailleur alignment. Using the big chainring up front might be putting the chain at an angle the derialleur cannnot accomdate. But you need to see if this is happening when you coast or just when the chain is moving.