I think you may have posted in the wrong forum. I Googled the coach model you stated and the Itasca brocure shows an Evolution chassis with a 350HP Caterpillar C7. Nothing Workhorse related there.
To answer your question, it sounds to me like you're hearing the waste gate on the turbo charger bleed off pressure. This sound is often heard on race cars with large turbos, when the driver lifts off of the pedal there's a squeal that immediately goes away once the accelerator pedal is pressed.
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Mike
2003 Alpine Coach 40MDTS - 400HP Cummins ISL
Can Am Spyder Roadster
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