So i have a super clean fully loaded 06 S60. Had tranny problems got it replaced. Have driven It for the past 7,000 miles like brand new. Wanted to go faster and rip on more Benzos and beamers lol got the polestar upgrade and wow NIGHT&DAY just rips soo smooth and totally safe for the car. A pretty penny but worth it. After and a very few times before its shakes ever so softly but I notice it cause I love and care for my car. Very intermittent tho. I've read about warped rotors and others but can the Motor mount starting to fail cause this.? I grabbed on it earlier to and it felt a wee bit loose? Any others with similar problems.? By the way car just hit 67,000 miles. I'm second owner getting her back into perfect shape. And I meant shakes when braking after ripping at high speeds. But when u left off and re apply it's not as bad.

 

Pop in neutral and apply brakes. If shaking, most likely rotors,
Also possible: tie rod ends, bushings.