I had that problem on my 2000 bf90 about a year ago. The power tilt and trim made a funny noise (at the motor) and the motor had trouble going up and down.
One night, at the end of a charter, the motor wouldn't go down at all! I had to get out of the boat and loosen the screw by sticking the screwdriver through a hole in the side of the motor bracket. Someone referenced the screw as the one with a big slot which is correct because a regular-sized screw driver would have only stripped the screw. I had the right tools on-board and got the screw to loosen (if felt like it hadn't been moved after 18 years of being dunked in salt water). When the pressure was relieved the motor dropped down.
When the problem first presented itself, i researched it, and the tools necessary to fix it on the water include a long, over-sized (really big) flat head screwdriver (found one at home depot, but not Wal-mart) and a large pair of vice grips (to turn the screwdriver). I think I used a pair of channel locks over a cloth for the plastic reservoir cap the next day but can't really remember. Without these specific items on-board i'm not sure i would have been able to get the motor down that night on the water.
Next day i watched a video on youtube, went to Wal-mart, got some hydraulic tilt fluid, purged the old fluid (which was low and had air bubbles), squirted in the new fluid, tightened the cap, let the motor go up and down, added more fluid, did that a couple times, tightened the screw back and its always worked great ever since. I think that the old hydraulic fluid had just given out and/or was low on volume.
It was a bit of a messy job but sure beat a trip to the dealer or having to call tow boat that night. I'm sure this isnt the video i originally watched but it should work:
https://www.youtube.com/watch?v=JGbRQ7TZsj8
I added some hydraulic fluid and the outboard is able to go up and down by itself now (I need to help lift it before). Any ideas what might be causing the noise, or troubleshooting steps?