Hdi Egr hissing noise,

[lozz]

ive got a Egr hissin noise on Tick over, sounds like a small pyfon
if iput my finger in the valve and push up it stops,
its not Emiting any zorst smoke or loss of performance but why do they start to hiss do they have a diaphram of some sort,
or wouid a few taps with the lump hamer sort it

thanks inadvance