Took my 98' auto for an ATF change last week to my trusted garage which is about 40 minutes away. On the way there it developed a strange vibration when travelling between 30 and 40mph. This was the first time i'd driven it more than a few miles. Got the change done, and drove it to get valeted. It didn't produce the same fault on the way there, only a 10 minute drive. It stayed there over the weekend, and I picked it up today. Decided to go the scenic route home and give it a bit of a blast and lone behold 20 mins down the road when I had to slow down for a village, the 30-40 vibration came back. Engine revs don't fluctuate like the feeling would make me think they would, and can feel it through the steering wheel and the seat of my pants. I don't notice it if i'm accelerating through those speeds, just if i'm sitting at them under low load. Car had sat for 2 years before i bought it last month. TIA.