Yes, you must take the bushing out or at least take the bolt out to see how much the bushing is worn. Just expect that they will be completely worn out if your trailer is more than a year old and has been towed a decent amount.
However, the sound you are talking about doesn't sound like it would be coming from the bushings. It sounds to me like a sound coming from the metal wheel or brake housing rubbing against the hub or brake backing plate. This could be caused by the castle nut that holds the wheel on not being correctly tightened or some other problem in the bearing area. That is where I would look first. I know you said you did it already but I would take it apart and do it again to make sure everything is ok.
Regardless of where the sound is coming from, I would take at least one bolt out and look at the bushings. They almost all fail very quickly after leaving the factory. Replace with bolts with grease fittings as shown in the links in the post I liked to earlier.
The only other place I can think of that would make that sound would be the WD bars on the hitch letting you know they need some grease. Dry bars often make a screeching sound when you turn - especially the round type of bar. They need grease in the area where they insert into the hitch head.
Hope this helps a bit. Good luck. :)
Barney
