Buzzing noise but picture is perfect

Buzzing noise but picture is perfect

My Optima HD20 was in regular use for 4 years from 2013 to 2017.  I moved since then and it had been in storage since 2017 till now (2019).  I plug it in and the image quality appears to be perfect but is making a moderately loud buzzing noise. The strange thing I hold it up on its side (with projector lense up) the buzzing stops.  Any ideas what may be the problem?  And more importantly the fix!  (hopefully an easy one)

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