Many of the 2000 +/- Prevost had complaints of hissing plus hard to steer for a few seconds at idle. There is a butterfly valve in the steering box that is the guilty party underneath your drivers seat. This hissing and slight hard to steer is normal. Some steer boxes make more noise then others.

There is nothing wrong and pretty much just an annoying sound and slight hard steer.

Just to be safe take a look at your hydraulic fluid level is in the normal range, and the service tec replaced the cap on your black tank.