Knock on startup?

2004 Subaru Forester X Amazing car, never leaves me stranded My last car (mazdaspeed) spun a rod bearing and my forester is making the exact same noise for like 1 full second on startup (maybe 60% of the time). Very rarely, but occasionally when its cold, instead of the rpms shooting up after the starter is “done”, it just stays at like 50 rpms and takes like 2 seconds to shoot back up, while making the knocking sound. But the weird thing is, my engine is 100% healthy right after, never making the sound again. Is this just the common Subaru start up piston slap or something? Something to be concerned about? It’s been doing this since I bought the car 3-4 months ago but never caused any issues