Adjust the valve if you want, while monitoring the performance of the water emitters downstream. You do not want to comprimise the performance there.
The sound you are hearing is the PRV doing its job: Reducing pressure. On my back porch I have a PRV threaded to many tubes to water many hanging plants and such. It squeals on occasion when I turn the water on to nourish these plants. It's not bothersome as the valve is only open for 5 or 6 minutes.