A balance belt vibration is more noticeable as you describe above at higher
rpm's. It's a very high frequency vibration, about 8 times frequency of a
The front balance shaft is tricky to time on this engine. Lining up the
marks isn't sufficient because the sprocket drives the oil pump and 2:3
gear drives the balance shaft.
Unfortunately, significant assembly would be required to check, and
everything apart before would have to be taken apart again to retime.
The official procedure for timing the balance shaft in the service manual
is to insert a screwdriver in a hole in the block, but this is
unfortunately behind the compressor or exhaust or something else I don't
remember because I refuse to spend the ridiculous amount of time necessary
to pull whatever it is out of the way.
With the front balance sprocket marks lined up, turn the sprocket to one
side a little. If it wants to continue turning away, it's timed wrong.
If it wants to turn back to realign the marks, then it'll be timed right.