2016 LEXUS IS 350 Suspension Problems

I AM THE ORIGINAL OWNER OF THE CAR AND AFTER 18,000 MILES, THERE IS SUSPENSION NOISE COMING FROM THE REAR PASSENGER SIDE. A LOUD THUMP CAN BE HEARD WHEN DRIVING OVER SPEED BUMPS, SMALL POTHOLES, OR UNEVEN ROADS AT MODERATE SPEEDS. DRIVING OVER THE ELEMENTS AT HIGHER SPEEDS, YOU CAN FEEL THE IMPACT THAT'S CAUSING THE THUMP. NO NOISE FROM SUSPENSION WHEN DRIVING THROUGH IT AT LOW SPEEDS WHEN DRIVING ALONE, HOWEVER, THUMPING SOUND IS PRESENT AT ALL SPEED LEVELS THROUGH EVERY BUMP ON THE ROAD WHEN I HAVE PASSENGERS IN THE BACK SEAT.