Pop test is carried out on pyrolysis furnace of an ethylene plant. The furnace itself is a 1500 psig steam generator. Pop test is to ensure the PSV will relieve at the set pressure and able to resit properly. This is carried out when the furnace is on hot standby without any hydrocarbon.
For...
In the ethylene plant I've worked in, the flare has the same noise description as yours. The noise as TD2K explained is due to the atomizing steam (we used 300 psig steam).
I'm not sure the local regulation at your side but mine specified continuously flaring for more than 15 minutes is...