Allow increasing the software PWM frequency.

Marlin/Configuration.h

 // Increase the FAN pwm frequency. Removes the PWM noise but increases heating in the FET/Arduino
 //#define FAST_PWM_FAN
 
+// Use software PWM to drive the fan, as for the heaters. This uses a very low frequency
+// which is not ass annoying as with the hardware PWM. On the other hand, if this frequency
+// is too low, you should also increment SOFT_PWM_SCALE.
+//#define FAN_SOFT_PWM
+
+// Incrementing this by 1 will double the software PWM frequency,
+// affecting heaters, and the fan if FAN_SOFT_PWM is enabled.
+// However, control resolution will be halved for each increment;
+// at zero value, there are 128 effective control positions.
+#define SOFT_PWM_SCALE 0
+
 // M240  Triggers a camera by emulating a Canon RC-1 Remote
 // Data from: http://www.doc-diy.net/photo/rc-1_hacked/
 // #define PHOTOGRAPH_PIN     23

Marlin/Marlin.h

 extern int EtoPPressure;
 #endif
 
+#ifdef FAN_SOFT_PWM
+extern unsigned char fanSpeedSoftPwm;
+#endif
+
 #ifdef FWRETRACT
 extern bool autoretract_enabled;
 extern bool retracted;

Marlin/planner.cpp

     disable_e2(); 
   }
 #if defined(FAN_PIN) && FAN_PIN > -1
-  #ifndef FAN_SOFT_PWM
-    #ifdef FAN_KICKSTART_TIME
-      static unsigned long fan_kick_end;
-      if (tail_fan_speed) {
-        if (fan_kick_end == 0) {
-          // Just starting up fan - run at full power.
-          fan_kick_end = millis() + FAN_KICKSTART_TIME;
-          tail_fan_speed = 255;
-        } else if (fan_kick_end > millis())
-          // Fan still spinning up.
-          tail_fan_speed = 255;
-      } else {
-        fan_kick_end = 0;
-      }
-    #endif//FAN_KICKSTART_TIME
-    analogWrite(FAN_PIN,tail_fan_speed);
+  #ifdef FAN_KICKSTART_TIME
+    static unsigned long fan_kick_end;
+    if (tail_fan_speed) {
+      if (fan_kick_end == 0) {
+        // Just starting up fan - run at full power.
+        fan_kick_end = millis() + FAN_KICKSTART_TIME;
+        tail_fan_speed = 255;
+      } else if (fan_kick_end > millis())
+        // Fan still spinning up.
+        tail_fan_speed = 255;
+    } else {
+      fan_kick_end = 0;
+    }
+  #endif//FAN_KICKSTART_TIME
+  #ifdef FAN_SOFT_PWM
+  fanSpeedSoftPwm = tail_fan_speed;
+  #else
+  analogWrite(FAN_PIN,tail_fan_speed);
   #endif//!FAN_SOFT_PWM
 #endif//FAN_PIN > -1
 #ifdef AUTOTEMP
       block->acceleration_st = axis_steps_per_sqr_second[Z_AXIS];
   }
   block->acceleration = block->acceleration_st / steps_per_mm;
-  block->acceleration_rate = (long)((float)block->acceleration_st * 8.388608);
+  block->acceleration_rate = (long)((float)block->acceleration_st * (16777216.0 / (F_CPU / 8.0)));
 
 #if 0  // Use old jerk for now
   // Compute path unit vector

Marlin/temperature.cpp

   float bedKd=(DEFAULT_bedKd/PID_dT);
 #endif //PIDTEMPBED
   
+#ifdef FAN_SOFT_PWM
+  unsigned char fanSpeedSoftPwm;
+#endif
   
 //===========================================================================
 //=============================private variables============================
 unsigned long watchmillis[EXTRUDERS] = ARRAY_BY_EXTRUDERS(0,0,0);
 #endif //WATCH_TEMP_PERIOD
 
+#ifndef SOFT_PWM_SCALE
+#define SOFT_PWM_SCALE 0
+#endif
+
 //===========================================================================
 //=============================   functions      ============================
 //===========================================================================
     setPwmFrequency(FAN_PIN, 1); // No prescaling. Pwm frequency = F_CPU/256/8
     #endif
     #ifdef FAN_SOFT_PWM
-	soft_pwm_fan=(unsigned char)fanSpeed;
-	#endif
+    soft_pwm_fan = fanSpeedSoftPwm / 2;
+    #endif
   #endif  
 
   #ifdef HEATER_0_USES_MAX6675
   static unsigned long raw_temp_2_value = 0;
   static unsigned long raw_temp_bed_value = 0;
   static unsigned char temp_state = 0;
-  static unsigned char pwm_count = 1;
+  static unsigned char pwm_count = (1 << SOFT_PWM_SCALE);
   static unsigned char soft_pwm_0;
   #if EXTRUDERS > 1
   static unsigned char soft_pwm_1;
     if(soft_pwm_b > 0) WRITE(HEATER_BED_PIN,1);
     #endif
     #ifdef FAN_SOFT_PWM
-    soft_pwm_fan =(unsigned char) fanSpeed;
+    soft_pwm_fan = fanSpeedSoftPwm / 2;
     if(soft_pwm_fan > 0) WRITE(FAN_PIN,1);
     #endif
   }
   if(soft_pwm_fan <= pwm_count) WRITE(FAN_PIN,0);
   #endif
   
-  pwm_count++;
+  pwm_count += (1 << SOFT_PWM_SCALE);
   pwm_count &= 0x7f;
   
   switch(temp_state) {