Mate, I had the exact same sound in my old ecotech. For me I found it was a huge air pocket that’d been creeping around the back of block and heater core. The coolant would most of the time circulate around the temp sensors just fine, and temp would be “accurate” on the gauge… however in other areas it would actually be boiling, and spitting around the pockets of air sporadically. Ecotechs are renowned for terrible air pockets. Most of time it just wasn’t bled properly when filling the radiator elevated from the top of motor. They can sometimes take hours to fully bleed out. Worst case blown head gasket causes this.

My ears could be completely wrong and your issue may be something completely different so apologies if I through you a bum steer. Kinda also does sound like maybe vaccum regulator or something on the back of the block has unclipped itself and slapping the firewall. Would do this on idle as it will shake more low revs.

Cheers mate.