I saw this on VX220.org.uk but thought id reply on here! 🙂
Mine makes a noise very similar to this on overrun when it had its air leak and when i recently over-ported my throttlebody (giving the good old P1415 code). What i put it down to is that the throttlebody was too far closed and the engine is literally 'hunting' (to try to run with the throttlebody virtually closed). Im assuming that after a SC conversion, Courtenay 'fool' the MAP sensor so it won't throw such codes. It could be down to that?
Isnt the conversion warranted? It may be worth going to them first...