So, instead of trying to answer your question from a meteorological perspective, I decided to examine it from a travel perspective. And it was the travel sites that provided the best information. That's where the data is organized for a traveler. But even there, it took some digging. Since Penang wasn't available on Lonely Planet, I clicked instead on Kuala Lumpur and found annual weather patterns for the region.
Rainfall is variable and falls all year round.
The driest months tend to be June and July. There was the destination page. Rain is common all year, but showers don't last long and shouldn't slow you down much. A rainy season brought on by monsoons lasts from November through February on the east coast of the peninsula, from October through April in Sarawak, and from October through February in Sabah. The heavy rains can cause delays.