If it's a WFCO converter, that's prolly the fan as mentioned. Mine was making a groaning whirring noise for approx. 5 mins after start-up, then it hummed like normal. I don't cherish making emergency repairs knowing damn well those units are unreliable. I bit the bullet and bought a Progressive Dynamics 55 amp deck mount model with the Charge Wizard feature. The nice feature is that it's quiet until the cooling fan is needed. One less thing to worry about when away from home. Progressive Dynamics has a great reputation and I'd strongly consider replacing your converter if it's starting it's 'Death Knoll'
2010 Cedar Creek 5th Wheel 34SATS "The Beast"
2006 Ford F350 Lariat 6.0L Diesel
2003 Harley Heritage Softail "Hogzilla"
1988 Harley Softail Custom "The Fireman"
1986 Marriage to "Wifey" (patience of a saint)