Mid-Life Speed

In the last two weeks an annoying knock has developed on the passenger side bank. It appears to be timed with the cam, so I am suspecting valve train components ... maybe a collapsed lifter, etc. The problem is I can't localize the noise with my stethoscope to the upper head area. As a matter of fact, the noise is the loudest through the scope when I probe the AC compressor mount area on the block around the # 2 cylinder ???

Would you expect a collapsed lifter to generate noise in this area, but little or no noise through the upper head assy ?

Could I be listening to a "flapping" timing chain ?

Could it be the notorious "piston slap" even though the knock appears timed with the cam ? (checked with timing light)

Just a side note ... there doesn't seem to be any SOTP loss of power, and no change in gas mileage or any excessive oil consumption.

I am at a loss, and can't decide if I should tear into it or not (yet).

Opinions please !!!