Fellow tsikoteers, I think we have a thread na 'ata for this.
Pero to answer your question, a previous vehicle of mine had the same problem. What I did was to install a 15w noise suppressor between my head unit and the power source. It eliminated the hum and the crackle. This was not the most elegant and audiophile solution but it solved my problem. If I remember correctly, that noise suppressor/noise gate cost me about P 150-175. You can get in Banawe or Raon.
Check your speaker wirings and ground cables there might be either an exposed wire or loose connections, and have your alternator checked if it provides suffient charge. Do you hear a whinning sound only when you depress the accelerator? or Is it there all the time? How old is your car?
Also check battery connections and grounds in the engine bay baka corroded na. You also might want to consider installing a grounding kit. Try searching it here. dami ng threads dito niyan.
hirap nga mag-isolate ng ganyang problem. lalo na kung kumpleto ka na sa gamit. iisa-isahin mo talaga.
stock stereo ba yan? bakit naman nagkastatic? kadalasan ng alam kong system na may static, e naka ampli.
kadalasan v12 ang ang may gawa.
or alternator talaga.
minsan nasa ground din.
as your car gets older your wirings deteriorate... so ang power handling capabilities nito ay humihina, tapos pag stock pa connections mo, the radio is sharing its power(+) with other electrical components... happened to my 97 lancer... biglang nag ka static sa radyo... bought a noise supressor sa raon (daichi ata yun) nothing happened... so what i did was insert a relay to the red wire which is attached to the ignition... ayun solved agad... till now walang static... sometime the noise supressor works, minsan hindi... buti na lang kilala ko may ari nung store napalitan ko na lang ibang item yung noise supressor