Singing Snowmelt

Long time reader, first time poster. Have a commercial snowmelt system served by 8 150,000 BTU wall hung, fin tube, mod/con boilers. The boilers are cascaded together and individual units come online and drop off as necessary according to system demand. After two months of operation each boiler sings - high pitched harmonics - as soon as it hits approx 90% firing rate - outlet temps are around 80 to 90 degrees at this point. Once the set point of 130 degrees is reached the boiler modulates down past 90% firing rate to maintain a 20 degree delta and the noise goes away. System was thought to be mixed to 30% glycol but one last zone has not been brought online so it was discovered that the mix is around 45%. We were able to make the noise go away using larger more expensive boiler pumps but the calculated flow rate difference between pumps was minimal - approx 11gpm with the smaller and approx 16 gpm with the larger. We are in the process of lowering the glycol mix for the current system - last zone won't go online until next winter - and hope that helps because the cost for 8 of the larger pumps is not pretty and not sure we want even more flow going through the boilers. Thoughts?