Hi Scotty, when my wife and I got married we were given a 2002 Volvo V70XC with 235,000 miles. It's a great car, however I've noticed when cruising down the road at around 40mph the car will start to shake and make a rattling sound that comes from underneath the car. This problem doesn't happen at any other speed which leads me to believe it's something to do with the transmission but I can't replicate the problem when I switch to "manual" mode. Any thoughts?
To be honest the Volvo transmission before 2004 were really bad as they were GM designs I believe and that is a LOT OF MILES. I'm not an expert but I know those transmissions were bad. In the mid 2000's, Volvo switched to using Aisin (Toyota) transmission where before they used junky GM transmissions. Please correct me anyone if I am wrong.
First of all, your XC70 (or V70XC as you call it) is an all wheel drive vehicle. That means that it has a transfer case that takes some of the power that would normally go to the front wheels and sends it to the rear wheels as well. It also means you have an additional driveshaft connecting the transmission/transfer case to the rear wheels. More complexity compared to a standard front wheel drive V70 and therefore more things that can go wrong. Since your problem only occurs at one speed, I am guessing that maybe your driveshaft could be out of balance or if there is a bearing filled collar on the driveshaft, perhaps those bearings may be worn. I have known of many XC70 owners who have actually disconnected the rear driveshaft thereby making it a front wheel drive only vehicle due to issues with the transfer case/rear driveshaft system. BTW, I think (not 100% sure) your Volvo has 3 different independent fluid reservoirs for the transmission/transfer case that need to be serviced regularly.
In reference to the previous answer, it is my understanding that the GM 4T65E transmissions with the most problems were on the XC90 SUV 2.9LT6 (inline 6 cyl. engine) 2004 - 2006 models. This same transmission was used on the S80 sedan models with the inline 6 cylinder engine but due to its lighter weight they did not fail as early as the XC90 models (30 - 60,000 miles). AND Volvo did use the Aisin-Warner transmissions in many of the cars in the 80s & 90s.
Finally, find a good INDEPENDENT Volvo mechanic. A good mechanic will be able to pinpoint your issue fairly quickly and give you more than one option for repairs! You didn't talk about any other transmission issues you may be having but there is a lot of information on the web re: these years and models. Do NOT go to a Volvo dealer! 236,000 miles is nothing for a Volvo if it has been well maintained with regular fluid changes. I have owned several (6) Volvos and some of them went over 300,000 miles without any major engine or transmission repairs. Good luck!
