I have a 2001 Jetta TDI, 248k miles. It has run great for its entire life. Recently, it makes a random popping kind of noise, sounds like from the drivers side top part of the engine. Could be in the large diameter hose from the air filter to the intake manifold, or in the air filter itself. I was not able to locate the source of the noise using a stethoscope. Seems like it mostly occurs at idle when hot. The random nature of the popping indicates to me it does not seem to have to do with any particular cylinder. I opened up the valve cover and the cam lobes appear to be ok. Anything else I could check?