Hi Ben,

I've had my standard ST170 for a couple of months now and have noticed that it hesitates at certain rpm's. Having looked on the forum, it seems that this is common and something to do with the ECU mapping as standard. Mine sounds a bit like yours and gets better the higher the rpm. In a way its typical of an engine with a degree of 'tune' and way back when I used to tune Escorts and similar generation Fords you always accepted the more it was tuned, the more 'highly strung' they become!! The ST has fueling suited to wide throttle openings and I suspect yours feels much better above 4000 rpm.

I would recommend investigating the hissing though as any air leak on the manifold side of the engine is not good and will affect the running of the engine. Spray some carb cleaner around the hissing area with the engine at idle. If the engine speed increases you have an air leak or split hose that needs sorting. You could also take the MAF sensor out and have a look at the fine wire inside - do not touch it but you can spray a bit of carb cleaner onto it.