Cabin heating not efficient, the hot air is barely warm

[2volvo]

Looks like I am going to need some help from the knowledgeable board members with regards to my 2003 Volvo XC70 cabin heating problem
The problem is that the hot air coming through all the vents in the car is barely warm. When idling, for example at a traffic light is gets almost cold. It is warmer at higher speeds but well below the normal hot air it used to be, so the car cabin is never warm.
The air is still cold even when switching to manual with the fan at full speed, so the interior temperature sensor is not involved.
The coolant level was fine and I flushed the heater core with plenty of water until the rinse came totally clean. The input coolant hose into the core is hot, the coolant out hose feels cooler, but I guess it is normal, so flow of coolant through the core seems fine.
I also changed the coolant and did a flush since it was about time, but it made no difference whatsoever.
The coolant temperature is right in the middle and the coolant feels hot, so the thermostat operates properly.
Some time ago, a small rattling noise from behind the dashboard started. It is not the usual cabin fan bearings going bad.
The airflow into the cabin also seems less powerful now.
Any suggestions you might have are much appreciated.

[Volvoholic]

I'm assuming your vehicle has an automatic climate control system. If this is the case, the interior temperature sensor is always involved. The A/C system needs to know what the ambient temperature inside the car is to know if it should blow hot or cold air compared to the temperature you set. I would check to see if the operation of the A/C system is working properly. It sounds as if the cooling half of the A/C system doesn't know when to shut off (which would involve the cabin sensor). That would lead to a cabin temperature sensor being bad or a part in the temperature assessment system of incoming or outgoing air being bad. Then somehow check to see if all the parts involved with signaling the cooling system to shut off are working properly. If you run the system with the A/C button unlit and the air still blows cold on full heat (with the engine fully warmed up), there would probably be an issue with one of the parts I'm describing.

Other than that, I'm not the best knowledge base when it comes to A/C systems, I usually call my buddy in the Navy that does all the upkeep on the cooling systems for the galley of his aircraft carrier to get his input but I'm unsure of what part of the world he's currently in. Therefore, I can't really reach him lol