Hi there!
I noticed some strange throttle response in my S60 T5 of late, so I started poking around. Full disclosure, I have a K&N filter and I clean it about every three months. Anyway, I pull out the air filter and there's a rodent's nest in the air box! I had just recently moved to a new place that's apparently a squirrel wonderland. Since it was winter, I guess one of the guys had moved in to have a warm place to stay. It took over an hour to clean out the air box and the filter. Meantime, I went to Home Depot and got a roll of galvanized steel "hardware cloth" for $7 and snipped out a grid to go in between the air duct and the housing mounted in the unibody. The whole thing snapped together pretty easily and it seems like a good barrier to the rodent bastards. I will let you know how it goes!

 

That's a fairly common problem, and it's not isolated to Volvos. I have an '02 Ford Exploder where squirrels crawled into the air box, ate a ton of acorns, and discarded all the shells in the bottom. For a time I had an '98 Ford E250 where mice made a home in the foam pillow thing that Ford puts in the lifter valley between the heads and under the intake, I assume to quiet engine noise. Damn things decided to try some engine wiring for dinner. Took me a few minutes to figure out the electrical gremlin that resulted. I've heard many tales about rodents getting into cars over the years. Driving the vehicle frequently seems to be the only way to prevent rodents from taking an interest in a vehicle, and even that isn't foolproof.

Excellent idea using steel hardware cloth. It hardly impedes airflow and rodents won't chew through it easily.

 

I had a similar problem. My wife was complaining about her fan making noise in her Highlander. Found a mouse nest build out of what was once the cabin air filter :-)

 

I used to have the same type of problem with my Jags. That nice warm flat engine was a sirens song to small animals at night. I would find nests under the hood on top of the engine. I ended up placing a box of mouse killer in a safe flat spot under the hood. Normally if I find a mouse in my house I take him back to the lower 40 and let him go free. But in my car, when they chew the wires.....nope.

Had a ground hog reside in a little beater car, a Saturn. It sat for months at a time. When I went to move it, there was no electricity to the started. Opened the hood and saw two eyes peering at me and the groundhog had eaten the entire wiring harness.