Arduino Mega - Use multiple Timers

Hi there,

I have an Arduino Mega and I want to output a modulated 38khz Signal to a IR-Diode. I thought about using two timers, one for generating the 38khz signal and the other generating the modulation-frequency.
My code so far:

ISR(TIMER3_OVF_vect){
  
}

ISR(TIMER3_COMPA_vect){
  
}



void setup_timers(){
 // Timer 1 fpr generating 38khz
 // period: 26us
 
 // output to pb5
 DDRB = (1<<SIGNAL_OUT);
 PORTB = 0x0;
 
 // Clear OC1A on compare match, set OC1A at BOTTOM (non-invertingmode)
 TCCR1A = (1<<COM1A1) | (1<<WGM11);
 // set prescaler to 8 -> 2mhz clock
 // 0.5us per tick
 // mode 14 fast pwm, top: ICR1
 TCCR1B = (1<<CS11) | (1<<WGM12) | (1<<WGM13);
 // top = 26 * 2 ( 2mhz = 0.5us) = 52
 ICR1 = 52;
 // 25% duty cycle (52*0.25 = 13)
 OCR1A = 13;
 
 // Timer 0 for 1250hz modulation
 // period: 800us
 // at timer 3 ovf: start 38khz
 // at compare match B: stop 38khz
 
 
 // Mode 14 fast pwm, top: ICR3
 TCCR3A = (1<<WGM31); 
 // prescaler to 8-> 2mhz
 TCCR3B = (1<<CS31) | (1<<WGM32) | (1<<WGM33);
 // 0.5us per tick
 // top = 1600 (1600*0.5us = 800us)
 ICR3 = 1600;
 OCR3B = 800; // 50% on, 50% off
 // enable timer0 compare match interrupt B
 // enable timer0 overflow interrupt
 TIMSK3 = (1<<OCIE3B) | (1<<TOIE3);
}






// setup 
void setup(){
  setup_timers();
  interrupts();
}

// loop
void loop(){

}

Everything works fine until I enable the Timer 3 overflow and compare match interrupts in this line: TIMSK3 = (1<<OCIE3B) | (1<<TOIE3);Although leaving the ISR empty I get a modulated signal on the output pin (frequency is about 13,78 Hz).

Any idea why this is happening?
Greetings