Here are the most likely culprits:
* Timing Chain Tensioner (Front): The 4.0L Explorer's front timing chain tensioner is a common point of failure. If it's not functioning correctly (either due to damage during repair or pre-existing wear), the timing chain can slap against the cover, causing a loud rapping noise. This is especially likely if you worked near the front of the engine during the rear guide replacement.
* Timing Chain (Front): The front timing chain itself might be loose or damaged. While less likely than a tensioner issue, it's possible something was disturbed during the repair.
* Camshaft/Crankshaft Sensor Interference: Though less likely to create a loud *rapping* sound, if these sensors are incorrectly installed or damaged during the repair, they could create a noise.
* Loose or Damaged Parts: During the repair, you might have accidentally left something loose (a bolt, for example) that's now rattling. Double-check *everything* you touched.
What to do:
1. Do NOT run the engine further. Continuing to run the engine with a potentially damaged timing chain system could cause catastrophic internal engine damage.
2. Re-inspect your work: Carefully examine the front of the engine, paying close attention to the timing chain, tensioner, and all related components. Look for anything loose, damaged, or improperly installed.
3. Check for interference: Ensure that no parts are rubbing or interfering with each other.
4. If unsure, seek professional help: This is a serious issue, and if you're not comfortable diagnosing and fixing it yourself, take your Explorer to a qualified mechanic. Continuing to run the engine with this noise is risking significant engine damage.
The fact that the noise started immediately after the repair strongly indicates a connection. Thorough re-inspection is critical to prevent further damage.
