The '98 model year was a split year for the timing belt tensioner. Cars build earlier for that model year have an "automatic" or hydraulic tensioner. Cars build later for that model year have what's called a "manual" tensioner which happens to be temperature sensitive. In other words, when it is installed, depending on the temperature at the time, it is adjusted accordingly. I'm wondering if this might not be your problem. Could be that it is just a bit off, but by the time the engine really warms up the problems go away. This would explain the vibrations on the right side (if I'm reading you right, this would be the TB side, right?)- most likely scenario is that your timing is slighly off at lower temperatures. Guess my questions are, do you know if your car has the earlier hydraulic tensioner or the later manual tensioner, and how long has this been going on- since the TB was last changed? Or, is it possilbe that the belt never been changed? I know it's a bit early, at 185K miles to change the belt and tensioner again (if it's been changed before- suppose to be every 70K miles on your car), but it might be worth it to at least have this checked. As all of our FWD/AWD Volvo's have interference engines, if the tensioner causes the TB to break, that's the end of our engines, so don't delay in this.
I may be totally off in my theory above, but I may not.
Let us know what you know about the history of your car's Timing Belt and tensioner.
Regards, and Happy New Year!
'98 S70 T5 Emrld Grn Met/Beige Tons of Upgrades Mobil-1
'04 V70 2.5T Red/Taupe Some Upgrades Mobil-1
94 850 Sedan NA Drk Blue/Tan
'00 V40 Purple/Grey Mobil-1