Dog with very strong, irregular heartbeat.

[LeChien]

My 1 yr old spaniel has an irregular heartbeat that I've noticed when he's resting. It hasn't worried me because he's very fit, healthy, eats fine and is incredibly bouncy.

Sometimes when he's resting he seems to be very twitchy, and it's just occurred to me today that they're not twitches, but he feels like he has a very strong heartbeat which makes his whole body move, I just haven't realised what it is, so he's been doing this for a long time.
He's now sitting with me chewing a toy and it's not happening and his heartbeat feels regular, it's happening when he's asleep or very relaxed.

We have a vet appointment next week for his boosters, is it worth mentioning, or does this sound normal?

He's a springer x cocker.
I've taken a video of this happening, so I can show the vet.

Any advice?

I noticed this about my rescue when I first got him, felt a strong irregular heartbeat when he was sleeping against me on the sofa. Told vet at imms appointment.
Was told its normal for heart beat to change with breathing, when dog is relaxed breathing is deeper and change in heart beat is more noticeable.
He still checked dog's heart though, so definitely mention to vet.

Ok, that's reassuring, thank you :)