I've noticed a roaring sound at times after starting my gas fireplace. I assume it's gas flow making the noise. It does seem to quiet some after burning a few minutes. Could this be a regulator issue?
to start I’m not an expert or anything for that matter. But from my experience working at an oil refinery. A furnace will make a sound that I feel you’re explaining when it’s in a fuel rich environment (Not safe if this is the case). So I’d have to guess the airflow is not right creating a lack of oxygen. Without oxygen the fire triangle is not complete, so the potential for the flame to go out would exist or for excess gas being put out but not burned. If enough gas builds up and oxygen is introduced it could cause an explosive fire if an ignition source exists. So maybe check your damper or cut the fuel back some and see if things run normally after that. Again I’m not an expect, that’s just my guess. Hope it helps
Papatots is right, in suggesting that this is a potential hazard. Especially with propane. Propane has a flame speed of 32" per second, that's why propane appliances have an 11" water column (pressure). Fireplaces will often have plenty of secondary air to complete combustion regardless of the pressure but that doesn't mean you want any deviation from the manufacturers allowable tolerance. An easy thing to look for is called "lifting" not like hitting the gym, but the flame, if the flame has a visible gap between itself and the burner the pressure is likely to high.
Disclaimer In any case these things demand respect. Don't play around with fuel burning appliances unless you understand the dangers. Propane can be very explosive and a delayed ignition on a propane fired fireplace can easily result in a tempered glass grenade.