After killing an enemy ward, a friendly Zombie Ward is raised in its place.
Proccing Comet on Rumble with either [E] or [R] is likely to result in a hit thanks to the slow. If opting for Rylai's [Q] should become reliable as well. Thanks to Rumble's mana-less, try to abuse Comet by landing as many abilities as possible while it's on cooldown.
WhyThe Ultimate Hat
The Ultimate Hat is a great choice for ensuring you have [R] up for ganks and any fight that should arise.
Take Nullifying Orb in this slot if you find yourself against multiple AP threats.
Absolute Focus is a great rune on Rumble when playing aggressively and poking in lane with [E]. Teleports bottom will benefit from the extra AP when looking for kills.
For a more reliable option, take Transcendence for some free early CDR.
Scorch is rune that is heavily abusable in lane by Rumble with [Q]. The damage scales decently on him as well since he likes to stack Magic Penetration early.
If you think the game is going to get stalled out into 30-40mins+ take Gathering Storm instead.
Cheap Shot grants you a significant amoutn of increased damage thanks to your ability to repeatedly apply slows from [E] and [R].
Zombie Ward offers you a ton of extended lane safety so that you're able to shove and harass your lane opponent.