I have a Nikon 200-400mm F/4 and the autofocus is fast and precise as it always was but recently the motor started to howl/whistle (maybe to squeal). I heard that those big lenses need their motors serviced after a couple of years. I wonder if I should take it to service immediately or if it can wait a little bit longer.
The squeal is only present when the AF travels from really far to really close. So only when focusing over long distances. When used on a normal basis it is not present. The focus ring is very smooth as always but if I don't use the lens for a week there is also a light squeal when turning the focus ring fast but this goes away pretty quickly.
How long can I continue using the lens? Do I risk damaging it when I use it a little longer before taking it to service? What is the worst that could happen? I imagine that it might get louder over time but at this state I really don't see a big reason to invest the money right now as it is 99% of the time not present at all.