Dark transmission fluid is not good anytime.

I think on manuals 30,000 is recommended change interval.

On a automatics I would consider making it less. If you do a lot of short drives. In cold weather areas its makes it more important to change fluid.

Moisture in the fluid does not provide lubrication and is not compressed like hydraulic fluid.

When it made that noise. Was it like a short low rumbling deep between the seats?

I don't have automatic Volvo's. The manual electric overdrives make that noise. It is usually old fluid that causes the sliding clutch to hang up.

If you have the trans serviced. The fluid change will for sure help.

I recommend using LUCAS Transmission additive be added with the new fluid.

I have put it in my '86 manual that had developed a no shift after hot condition. I suspected that at 280k that maybe the seals around the clutch pistons were leaking by a small amount. Thinner fluid when at highway speeds.

The Lucas product appears to either slightly thicken the oil or make it more slippery. It works better but eventually I will probably have to tear down the overdrive unit for the piston seals.

With a manual, it only uses three quarts. I went with the recommended 20% ratio. The stuff is thick out of the bottle. I mixed it up with my replacement fluid prior to filling. The fluid thins the additive right out.

You may use a whole quart.

This might be a good maintenance thing. I doubt it can hurt. Again DARK is a indicator, worth a short thought in my mind.

Good luck,
Phil