Even when the car is stopped and parked with engine off, I hear this sagging noise coming from the engine bay, I believe is from the passenger side? What can the noise be? Just had the rubber boot joints replaced at 180K two months ago, but the sound is still there.

 

Check out your engine / transmission mounts, control arm bushes and your front strut mounts.
Something may be due for replacement.
Any noises whilst moving or on uneven road surfaces??