Hissing at startup & under heavy acceleration

98 Ram 1500 Sport. 78K miles.
My truck will hiss upon startup and under heavy throttle. If I allow the truck to warm up a little, the sound will go away at low RPM, but
return under heavy throttle.
If it helps, this seemed to pop up after an oil change.
Any ideas ? I hope vacuum pump is okay. Those are pricey.
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