When you tap the GPS it is probably doing one of two things:
1. The tapping is temporarily reconnecting the broken voice coil in the speaker (speaker will need to be replaced)
2. The tapping is temporarily reconnecting a bad solder joint from the GPS motherboard to the speaker. (The solder joint will need to be resoldered)
You can either send the unit back for repair or purchase a FM modulator and retrieve the audio through your FM car stereo.
You can purchase an external FM modulator like those used on IPODS and CD players. I am using one on my Nuvi 350 and it works well. Just plug it into the headphone jack. Advice: Be sure to buy one that is crystal (PLL) controlled and NOT using a tuning dial. The FM modulators that use a tuning dial often tend to drift and they will also change frequency when you hit a bump in the road.
If you decide to send it back for repair, be sure to have your serial number handy. Register it online https://my.garmin.com/mygarmin/marketing/register.htm
if you haven't already done so.
Monday - Friday
8 a.m. - 7 p.m.
Central time (closed holidays)
1-866-429-9296 (Canada) or (913) 397-8200