Hissing noise after shock service on my S550, is this normal?

[howardprescott]

Took my 2018 GT to a Dubai Mustang Shock Absorber Service last weekend to swap out the rear shocks because they were leaking, and the car feels planted again but now I've noticed a faint hissing sound from the rear when I go over bumps, almost like air escaping. The shop said it might just be the new shocks settling in, but I'm a little paranoid because I've never heard that on any other car I've owned. Has anyone else experienced this after a shock replacement on their Mustang, or should I take it back to have them check for a loose fitting? Just want to make sure it's not something that'll get worse.