The most likely cause of this is that you are driving too fast past a target. Turrets in vanilla cannot aim quickly enough to shoot fast moving targets, even after locking onto them. This also applies if the turret is moving past a target too quickly. I developed the Better Turret Aim plugin in conjunction with this to address that problem for those who want it (also works for stationary turrets and those on other vehicles). Keep in mind that allowing turrets to shoot players while driving by very fast may become a balance issue.
i literaly stop the car and it was locking onto a sientist but wasnt shooting..had a ak in it with 500 bullets...tryed several weapons and same results..im also on a modded server...all turrets work fine but car turrets
Do you have any other plugins that affect turrets? Have you also tried multiple types of targets, besides just scientists?
To my understanding, a turret will beep when it loses a target it was locked, which can happen when it thinks the target is out of range or if there is no line of sight. How the turret perceives those could be affected by a plugin or some other vanilla behavior manifesting in particular situations.
Feel free to send me your Discord and I can try to debug more closely with you.