I have it stuck in my head, but I can’t remember what it’s from. It sounds like the end of a big Russian orchestral piece. It’s a bit like the end of The Great Gate of Kiev, but it’s NOT the same theme.
It consists of the same phrase repeated three or more times, each time with different orchestration and becoming grander:
Maybe Shostakovich’s Symphony #7 (Leningrad)? The first movement has a theme that repeats a bazillion times, starting with a single instrument and adding more in. The theme doesn’t quite seem to fit your rhythmic structure, though, so I’m doubtful.
The first thing that came to mind was one of the themes from the 1812 overture by Tchaikovsky. But assuming you’re an American I figured you’d already know that note for note…
ETA: You wanna whistle it into a mic? That would help so much with this!