Time to replace both the battery and the power adapter (easy to replace). Quite possibly the DC power jack (requires removal of motherboard and minor soldering). Considering the age of the laptop, motherboard repairs could be a disaster.
The condition is only going to get worse and possible dangerous.
Easy to answer.....
If this laptop was brought to me for repair: In order of importance.
1. Replace battery
2. Replace power adapter
3. Replace DC power jack
You can replace the battery and power adapter yourself ( I wonder why you haven't done that already?).
If noise persists then proceed to have DC power jack replaced.
Not sure if this will make sense and you will be interested, but have heard some G74 customers before suggest using the 180W adapter instead of the 150W............