You may want to be a little more specific like what part of the dash? Is it the right side or left side?

I had a little squel coming from the right side on my 88 190e and what it was, was a air pump for the temp gauge of the cabin. Located on the right hand side of the dashboard, behind/beneath to the right of the glove box. Put a little trans fluid on the bearings and the squeal went away.

If it is a vacuum leak and it is serious that it makes lots of noise, do all the vents of the climate control function properly?

Iggy

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Insanity: doing the same thing over and over again and expecting different results.

2006 - Suzuki Gran Vitara (2.0 L fully equipped) Like this car so far except for trying to put on the seatbelt.
1988 - 190e - 2.3L - 172K miles (It now belongs to the exwife)
1999 - Chevy Blazer LS Fully Equiped - killed it June 2006
2001 - Honda Civic EX - 68K miles (sold June 2004)
1963 - 220S - Dual Carb 6 cyl. (sold)
1994 - Yamaha WaveRaider (fun to ride)