Volvo: 740 turbo engine noise, spark plug wire, head gasket

Question
I have a 1990 740 turbo wagon with 237K miles.
The engine is making a noise that sounds like an exhaust leak (blop blop blop).The frequency changes with engine speed. This noise stops when the #1 cylinder spark plug wire is unplugged.  I checked the compression in all cylinders and all are at 150.

I can find no exhaust leak. I also checked the valve shim clearance and all are within spec.

The engine has never had any internal work done on it.  I am the original owner. The only repairs have been a water pump, belts, hoses, fluid changes, and other small normal stuff.

Any ideas?  

Answer
Have you checked the timing?  The blop kind of sounds like post ignition knock, meaning the timing is too far advanced.  Does the sound get worse under heavy acceleration or going up hills?  If your timing is within specs, could you give me a more thorough description of the sound?  Also, check for oil in the coolant and water in the oil, this could be a head gasket breach at the #1 cylinder.  Have you noticed a change in performance, even slightly?  My Volvo had a slow gasket leak that didn't show up on compression checks, but caused weird ping noises as well.

Jason