The heater in my 2006 frontier seems to run cold when I'm setting idle what would cause this

your a/c compressor is puttin load on engine and a/c runs with heat also to dry air. have system ck for low 134 or problem a/c compressor .

a question to an answer: is your truck has any heat when you rev it 2000 rpm, or above?
have you check your coolant level? if level is low you have a small coolant leak, a water pump may be at fault, and causing air pockets in your heater core, check for leaks your self, dealer wont tell you if vehicle still under warranty.
good luck

There is a diversion valve that closes off the heater core. If there is an air pocket or if pressure bypasses that could cause the thump. And allow the heater core to warm up and in turn cause the ac not to blow as cold. It is like having the heat and the ac on at the same time. To fix this make sure cooling system is free of air, and check the diversion valve to ensure proper working.

It could be your thermostat inside your radiator hose. They are a common thing to fail, but luckily relatively cheap and easy to DIY with some automotive background.

Thermostats usually work with a spring that allow them to have a closed "door" while your car is still cold working its way to optimum operating temperature. (Usually around 200°F depending on vehicle. You can check with a parts store for your thermostats rating) Once hot the thermostat opens to keep the engine from continuing to get hotter to the point of overheating.
When a therm. gets stuck in the open then your vehicle will usually not allow cabin heat in efforts of keeping emissions down. Cold running engines produce more hydrocarbons = not environmentally friendly.
This could also trigger a code that could set your check engine light. Before you go changing anything you should stop at an auto part shop where they can check what codes are stored for free. Codes aren't black and white though so do some research after as well.

A bad thermostat would set codes such as

•P1221
•P0128

jtg

no i think the water pump impeller is starting to spin on the drive shaft ,the impeller is plastic on a metal shaft ,fairly common fault

You need to bleed the cooling system of excess air and add aditional antifreeze.

This sounds like your automatic climate control is starting to malfunction. Check to make sure your coolant is full, and temp is not fluctuating, and when your vehicle is warmed up, try using the low setting and feel for air temp. Is it warmer than if you turn it up? When at idle the vehicle is not producing heat the same as if you are driving, putting load on your engine and more air/fuel combustion means more heat, which is why you can feel heat while driving. Ideally you should feel heat as soon as the engine is warming up, getting hotter as it reaches peak temp of 190-210 degrees. You can try to make sure your water control valve is not blocked, and all electrical connections are secure. This will be located close to your firewall and connected to the coolant tubes that run into your firewall to your heater-core. Also use recirc (setting)to try to summon as much heat as possible, however ultimately, you may need to take you vehicle in for service. If it only warms when driving defrost will be an issue. Good luck.

it takes quite a while for a diesel to warm up by just idling in order to produce any heat.you have to put them under a load (drive it), before you will get any noticeable heat.if you have a block heater, plug it in 4-5 hours before you plan on driving it and that will definitely help produce interior heat quicker.

Impalas are known for this issue, where the heating won't work properly.... usually caused by trapped air/bubbles in cooling system or slightly low coolant level. Park car so that the right front side is elevated and remove the coolant OVERFLOW bottle cap (NEVER the radiator cap) while engine is running.. This should allow the trapped air to escape. Also add a bit more coolant if needed.