Hot water heater

so as of 4 months ago my 10yr old Polaris hot water heater started making a thumping sound only heard outside on the exhaust pipe when the burner starts. After it’s been running the thumping sound stops. But once burner starts again the thumping starts again. I took the cap off the condensate line and when the burner initially starts water splashes out the top. I have attached some pictures to see what your thoughts are. The condensate line was blown out about two week ago only from the white pvc cap