Sorry to resurrect an old thread but just wondering if the OP solved this noise problem in the end?
I've had the same thing on a 2009 manual 1.2 Vtec Mk2 Jazz for well over 18 months and I just can't seem to pin it down. The noise only appears in 4th and 5th gears between 1600 and 2000 rpm with moderate to heavy throttle and engine under load (but even then it will do it on the flat or slight downhills). It's most noticeable when accelerating up to 40 at the end of a 30mph zone in 4th gear.
So far I've tried 99RON fuel, new spark plugs, Italian tune-up, checking and adjusting the valve clearances, Redex added to the fuel and new manual transmission oil along with the standard service items. None of these have had any effect on the noise problem but did make the car feel livelier.
I can only think now that it's either a loose heatshield, slight manifold leak, a gearbox bearing problem or perhaps a clutch problem.
It'd be great to know if the OP or anybody else has found the fix as it's been driving me crazy