My Vitz 2004 (only petrol) when accelerated from stop position, it produces a strange whistling sound which increases a bit further when the gear changes for the first time and then sound remains a bit high until speed of 40 and then the sound almost goes as speed increases further. At higher speeds you can't really observe this sound. The sound only comes when you press accelerator and if you lift your foot from accelerator while driving then even on lower speed the sound immediately goes. There is no such sound when car is in N or P gear. I feel this sound almost at front of driver side in engine area. I want to ask:
1- What could cause this sound (it has been driven ~10K after transmission oil changed with ZIC ATF, the engine oil and air filters are just changed last week)
2- Is it related to engine? transmission or exhaust system?
I would like to add that I started to hear this sound after I replaced the brake servo as the older was causing the sudden hard braking. Is the new brake servo may have any connection with the sound?
Thanks in advance for your advice.