Thep, you learned a valuable lesson. We learned the hard way as well. Remember, you are the customer. You are spending a huge amount of money on a product. You are entitled to a thorough test drive under the conditions you plan on operating the vehicle. You dictate the test drive route.
If a salesperson refuses to allow you to take a particular route it is probably what his manager has told him to do. So, talk to the General Manager of the dealership, not just the sales manager. If that doesn't work leave. We have done this several times before.
Also, we research a lot on the net as well as talk to owners we meet. Research brought up comments on the noise so we expected to hear it on our test drive.
I just had A/T tyres installed and now have that noise as well. A real rolling symphony.