You need to put a tiny chamfer on the outside of the air holes. Mine did the same until I took it into work, clamped it in a pillar drill and ever so slightly countersunk the air holes.
Having looked on places like knucklehead it seems like some replacement caps are sold with chamfered holes expressly to stop any whistling.
I can tell you that the tiny chamfer I put on the cap completely stopped the awful whistle.
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It worked. Cheers.You need to put a tiny chamfer on the outside of the air holes. Mine did the same until I took it into work, clamped it in a pillar drill and ever so slightly countersunk the air holes.
Having looked on places like knucklehead it seems like some replacement caps are sold with chamfered holes expressly to stop any whistling.
I can tell you that the tiny chamfer I put on the cap completely stopped the awful whistle.
View attachment 152548