Yellowing of needles on noble fir can be caused by several conditions. Here is a list of several conditions and insects that could be the problem. I would recommend taking a sample to your local Extension Office for exact evaluation.

Mechanical damage: Injury to the tree’s cambium can occur from tractors, lawnmower or other heavy equipment.

Drought: All Christmas tree species, especially Noble fir, can be stressed by drought. Plants growing in gravelly or sandy soils can show symptoms of wilting of new growth, top dieback, tree death. Other signs can be loss of interior needles, shortened needles, needle tip dieback, overall slow growth, and death tree. Symptoms generally start at the top of the tree and continue downward. Damage may occur over several years. Trees weakened by drought, may lead to other problems, especially insects and disease.

Engraver beetles attack Grand fir, noble fir and true firs they are vulnerable, all sizes. In stressed trees attacks from fir engravers may result in: (1) dead branches, (2) top kill, or (3) complete tree mortality. Needles on attacked portions of trees turn yellow-green and eventually red. Some trees fade in the fall after an attack, and others may fade the next spring.

Root Weevil Otiorhynchus sp.: Trees damaged by this weevil include Douglas-fir, noble, grand, most Christmas tree species. Reduced plant growth, yellow needles and premature needle loss are some symptoms. Other signs are scalloping or notching along needle margins. Larvae are legless grubs, bend their bodies in the shape of a letter “C”, can cause both needle and/or root damage.

Eriophyid Mite Trisetacus spp.; Epitrimerus pseudotsugae, and Nelepella ednae. Noble firs as well as other firs can show damaged needles either yellowing and stunting of new needles, or yellowing and curling of more developed needles on new shoots. These affected needles later turn brown, die, and drop from the shoot, leaving naked branch tips. Needles at the branch tip may appear white flecked or fuzzy when population are high.

References: https://www.oregon.gov/ODF/Documents/ForestBenefits/Fir%20Engraver%20Beetle%202017.pdf

https://agr.wa.gov/PlantsInsects/PlantServicesProgram/PSPInspectorManual/docs/ChristmasTreeDiagnosticFieldBook.pdf

https://catalog.extension.oregonstate.edu/sites/catalog/files/project/supplemental/pnw659/ pnw659.pdf