Thunder (천둥, cheondung) is read primarily as an announcement — sound without necessarily any visible danger, which is part of why it's interpreted differently from lightning even though the two usually happen together in real weather. Traditionally, thunder is associated with heaven's voice or judgment, a message arriving rather than a threat striking.
Distant, rolling thunder is read as news on its way — often sudden, sometimes startling, but not automatically bad. Loud, close thunder that genuinely frightens you in the dream is read more seriously, closer to a warning that something significant, and possibly unwelcome, is about to be announced or revealed.
Thunder without any rain following it is sometimes read as a warning that doesn't fully materialize — noise and alarm that turns out bigger than the actual event. Thunder that leads into a full storm is read as the announced matter genuinely arriving in force, tying the dream back into whatever the accompanying rain or wind is doing.
Because thunder is fundamentally about sound and suddenness rather than a fixed image, interpreters generally treat the emotional intensity of the dream — how startled or unsettled you were — as more informative than the thunder symbol on its own.