I'd say turbo related, even if it is not burning oil and runs like a top. I say this based on the 98 XC I bought, purchased short cash with about 192k on it (crazy you'd say?)....but in all seriousness I had the same issue and symptom with that car and drove for 30k miles with absolutely no degradation of engine performance.

My 07 has started to develop this noise oh so slightly, and she just hit 108k. After getting to operating temp its non existent and I've used Mobil 1 since purchase a few years ago.

I do believe reading up on an issue where the turbo, or some element of the exhaust in close proximity to the turbo suffered some expansion and contraction issues after a period of time, no harm if left unattended to though. I would double check the rubber gasket/hose that connects the boost pressure pipe to the turbo itself, check for looseness and cracks, may require an remove and inspect to makes 100% sure. Remember, this car, although a LPT still boosts up to about 8PSI at minimal RPM, even a small knick may cause such a sound.

General turbo test though, pull fresh air inlet house of any give a wobble to the turbine assay, inspect for abnormal movement. While there may be a great time to pull the PTC orifice assy and clean.

Good Luck.
Steve