My girlfriend has a 98 civic lx (i think). It has started to randomly make this humming noise( sounds like a vibration but you dont feel anything), it starts quielty gets loud then stops completely and it onyl takes about 3 seconds for it to happen. It has only happened on longer drives, after aout 100 miles of being on the road and it only happens between 30-45 mph once it starts it will do it every so often. It has done it once in city traffic ( not after 100miles) but it was raining. If anyone has ever had simular experiences or knows what this could be I would really appreciate it. If you need more info ask away.

thanks

 

can you distinguish where the noise is coming from- cab, front left ect.

 

It seems to be coming from the center right behind the dash board. Someone mentioned a valve in the A/C system that can cause this. Any idea if that would be true?