The only large diameter piping that I can think of is the intercooler (IC) system. The pipe goes from the turbo, close to the firewall, over the top of the engine to the intercooler and then back to the throttle body from the bottom of the IC.

Depending in the mileage/age, it is possible that the seal on the turbo is leaking oil into the IC. This used to be 'normal' on aging 5 cyl engines whenever boost is generated. The 'fix' is to remove the turbo and replace the rubber seals. Not a cheap thing to do, but a good mechanic can do it at home for around $150 in parts.

A Volvo dealer will only replace the turbo for an astronomical fee.

One thing to check is to remove the air cleaner to turbo pipe, exposing the top of the turbo vane, and checking it for looseness using a finger.

The worst thing that one can do to a turbo is to shut the engine off immediately after full boost, not allowing 60 seconds for the turbo to cool off. This happens to a lot of XC90s with the twin turbo setups, destroying the turbo vanes and making the replacement of both turbos cost more than the car is worth.
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Keeping it running is better than buying new