Reibun
。

The thunder rumbled with such a crashing sound that lightning seemed about to strike at any moment.

Literal

Thunder [subject-が] right-now-[adverbial-にも] fall-seem-to [attributive-な] violent sound [with-で] rumblingly [adverbial-と] sounded.

Built around an embedded clause 今にも落ちそうな ('seemingly about to fall at any moment') modifying 激しい音. ごろごろ is a 擬音語 (sound mimetic) for low rumbling — thunder, a stomach rumble, a cat purring. The と after ごろごろ is the optional adverbial particle for mimetics.