135 Lehman Air Sound

At start up one day I noticed a pressurized air sound comimg from the intake manifold. I was able to isolate it to the 4/5 port of the intake manifold and it was a rythmic pressure sound that picks up speed with RPMs. Even at cold start up I do not see exhaust smoke and the engine did not overheat so I dont think it is exhaust leaking. After completing a compression check all checked out at 380 to 420 with the 4/5 being 400 and 410. All 6 cylunders are firing and the motor runs smooth and normal other that the sound. Has anyone seen this before and is it possible to have these compression numbers and still get some blow by through the intake valve creationg this sound???? Thanks

Ken, it is possible to get good readings and still have a pin hole leak at the valve seat. All it takes is a little piece of carbon on a valve stem or seat. If you continue to run damage to the valve and seat will become worse. Check your valve clearances you may have a tight valve.
How many hours on the engine and were the valves ever adjusted ?? .015 intake .012 exhaust

Charlie,
Thanks for your response as this is my first diesel so I took a basic class but it is all still new. The engine has just over 2000 hours and prior owner appeared to be a good maint guy with good records and boat condition. I had the valves adjusted when I bought her in Dec and I have had them checked since the sound appeared. I thought the same thing with a seat or carbon junk so I guess the next step will be pull the head to be checked at a machine shop. Do you agree???
Thanks again

Ken, you might try removing the valve cover and rotating the engine by hand (breaker bar on front of crank) untill both intake and exhaust valves on the cylinder you suspect have clearance between stem and rocker arm. Gently tap the rocker just over the valve stem with a hammer several times. The shock of sudden seating may dislodge the culprit. 2000 hrs isn't much for a lehman and sorry to say if this doesn't work head must come off !
Hope this trick works for you.
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Re: 135 Lehman Air Sound ATTN Charlie

Charlie,

Since you were kind enough to respond with info I thought I would pass on the results. I removed the head and took it to a machine shop. After a tear down we discovered that the #5 Intake valve was cracked 3/4 of the way around on the back side of the face of the valve. This must have caused a poor seal letting just enough compression out to cause the sound but still holding enough compression to read within range. Used this tear down to replace all valves (guides were good) seals, hoses, gaskets, test injectors and new fluids. Engine is now back together and running like new. I was very lucky the valve head did not break off and cause damage to piston/Cylinder etc.

Thanks for your information,
Ken