Beer foaming from Barrel after aging

I brewed an Arrogant Bastard clone... 2 weeks in primary. 3 weeks in secondary. Now 3 months in an eight-gallon whiskey barrel (first time it's been used - received from distillery - I soaked with water and full bottle of whiskey after receiving).

I left it sit quietly since January and I wanted to transfer to keg today. (I wanted to age longer but I have a party in two weeks that I want it for). It has been sealed with the original wooden bung.

I tried to remove the wooden bung / stopper just now and upon hammering it loose, the bung hole immediately began foaming. Clearly it's under a lot of pressure. It I hammered it any looser, beer / foam would begin pouring out. I pounded it back in to stop foaming.

What should I do to relieve the pressure so I can transfer to kegs?

Currently I have the bung slightly loose in hops the pressure will release slowly without losing anymore beer.

Next time I'll use a drilled stopper with an airlock to be safe. My fingers are crossed that I have some excess normal fermentation happening and no infection. My OG was 1.070 and SG before tertiary (barrel) was 1.019. I obviously can't check FG until this foaming subsides.

Any suggestions aside from slowly allowing it to leak? Thanks!

After a few hours of slowly leaking out the gas, it stopped and I transferred the beer. All good.

The gravity dropped to 1.009 and it seems a little thin but NO problems.

As always, RDWHAHB.

I have a barrel doing something similar. Did everything work out? I am going to bottle mine and I'm worried I might have something eating all the stuff the normal yeast left behind and I'll have some bottle bombs.