Here's why:
* Rear Main Seal Leak: This indicates the transmission is losing lubricant. Low transmission fluid can lead to increased wear on internal components, resulting in noises and potential damage.
* Clunking Noise: This points to a worn or damaged component *inside* the transmission. Several possibilities exist, including:
* Worn Bearings: Low fluid exacerbates bearing wear, leading to clunking as they're not properly lubricated.
* Damaged Gears: Similar to bearings, gear teeth can be damaged due to lack of lubrication, creating a clunking sound.
* Shift Fork Problems: While less likely to be the primary cause of the clunk *when stopping*, a damaged or worn shift fork can contribute to issues with gear engagement and might manifest as an odd noise.
What you should NOT do: Continue driving the truck. Driving with a low transmission fluid level will cause catastrophic damage to your transmission.
What you SHOULD do:
1. Top off the transmission fluid (carefully): Use the correct type and weight of transmission fluid for your truck. This won't fix the problem, but it will slightly mitigate further damage until you can get it repaired. Be cautious; if the leak is substantial, you might not be able to keep it topped off for long.
2. Get a professional diagnosis: Take your truck to a reputable mechanic specializing in transmissions. They can properly diagnose the source of the clunk and the extent of the internal damage. They'll likely need to inspect the transmission's internals after removing it.
3. Prepare for a repair or rebuild: Depending on the extent of the damage, you might need a transmission repair (addressing specific worn parts) or a complete rebuild (replacing many parts). A complete replacement is also a possibility.
The clunk is a serious warning sign. Ignoring it will likely lead to a far more expensive repair later, possibly rendering your transmission unusable. Get it checked out as soon as possible.
