It is most likely a white fuse, probably #5 (on my 87, your 90 may be different, look for a fuse that controls the instruments:



The fuse develops a hairline crack, and makes rapid intermittent contact, causing the buzzing component. I learned this little tidbit here from these great folks 2 years ago when mine did the same thing. This fuse also controls your brake lights, which may not be working either, depending on how rapidly the fuse is cycling on and off. Even if the fuse looks good, replace it. Regards,

__________________
John

2003 Firemist Red/grey leather SL 500
2015 Palladium Silver/black mbtex GLK 350
1987 Smoke Silver/burgundy mbtex 300E Sportline (SOLD)

Click to see 87 300E