Ok, here's how it turned out. Since posting my question I bought a can of "Engine Flush" at the local Autozone and also picked up a case of Mobil 1 5W30 and swung by the Volvo dealership for a couple of OEM filters at $12 each. Ouch! I had installed a Fram when I changed the oil first time to
Mobil 1, 10W30.

First, I should say that the noisy lifter had been clacking constantly for several days and was no different when I driving around picking up this stuff. Of course by the time I got home it was much quieter. I put the car up on the ramps, drained about a quart of the 600 mile-old oil out and put the Engine Flush in. I'll say the color of the oil coming out was like a dark rum. As per directions, I let the engine idle for 5 minutes with the flush in and the lifter clacked the whole time. I was kind of nervous about all this so I sat in the car watching for an oil pressure light.

After the 5 minutes, I drained the oil and it was BLACK. I also drained the oil cooler. I filled the new filter, installed it and torqued the drain plug to 26ft/lb with a new aluminum washer.

In goes the 5W30 (good to 68 F according to the manual), check for leaks and drive. Very quiet! A little quicker feeling too! I've just driven in to work and there was no lifter noise at all. Keeping my fingers crossed. BTW and FWIW, using the screwdriver stethescope it seems it was an exhaust lifter, near the center cylinder. If the noise comes back, I'll go in and replace the oil pan O-rings.

Thnaks to all who responded, your input is greatly appreciated.

Erwin in Memphis