Mazda 6 Service Manual: No.20 noise occurs at idle when vehicle is stopped in all positions/ranges

No.19 excessive shift shock on torque converter clutch (tcc)

No.21 noise occurs at idle when vehicle is stopped in d, s, l ranges, or in r position

Other materials:

Mazda 6 Service Manual: Foreword
When the customer reports a vehicle malfunction, check the malfunction indicator lamp (MIL) indication and diagnostic trouble code (DTC), then diagnose the malfunction according to following flowchart. - If a DTC exists, diagnose the applicable DTC inspection. (See DTC TABLE.) - ...

Mazda 6 Service Manual: Heater core inspection
1. Remove the A/C unit. (See A/C UNIT REMOVAL/INSTALLATION.) 2. Remove the heater core from the A/C unit. 3. Inspect for cracks, damage, and coolant leakage. If any problems are found, replace the heater core. 4. Inspect for bent fins. If any are bent, use a flathead screwdriver to stra ...