Chronic orchialgia: This condition is characterized by persistent testicular pain, often without any identifiable cause. The normal scrotal US and presence of a trace hydrocele on the left side do not necessarily point to a specific cause, making chronic orchialgia a plausible explanation for the ongoing pain.

Epididymitis: Although the scrotal US is normal, epididymitis can sometimes present with minimal findings on imaging. The presence of a hydrocele could be related to inflammation in the epididymis.
Varicocele: A varicocele is an enlargement of the veins within the scrotum and can cause chronic testicular pain. However, it is usually associated with a palpable abnormality, which is not mentioned in the scenario.
Spermatocele or epididymal cyst: These are benign cysts that can cause discomfort or pain, but they are typically palpable and may be visible on US, although small ones might be missed.

Testicular cancer: Although testicular cancer is less common in older adults, it is crucial not to miss this diagnosis. Pain is not a typical initial symptom, but it can occur, especially if the tumor causes bleeding or invasion into surrounding structures.
Testicular torsion (chronic or intermittent): While testicular torsion is more common in younger men and typically presents acutely, chronic or intermittent torsion can occur and might not always show typical findings on US.
Inguinal hernia: An inguinal hernia can cause testicular pain due to pressure on the testicle or surrounding structures. The pain can be chronic and might not always be accompanied by a palpable hernia.

Henoch-Schönlein purpura (HSP): A systemic vasculitis that can cause testicular pain among other symptoms. It's rare in adults but should be considered in the differential diagnosis of testicular pain.
Polyarteritis nodosa (PAN): A systemic vasculitis that can affect multiple organs, including the testes, causing pain.
Sarcoidosis: A systemic disease that can affect any organ, including the testes, leading to chronic pain.

Given the normal scrotal US and the presence of a trace hydrocele, further workup could include:

Physical examination to assess for any palpable abnormalities, including varicoceles or hernias.
Urinalysis to rule out urinary tract infections or other conditions that might cause referred pain.
Consideration of tumor markers (e.g., beta-hCG, LDH, AFP) if testicular cancer is suspected, although these are not typically elevated in the absence of a palpable mass.
Imaging of the abdomen and pelvis if an inguinal hernia or other intra-abdominal pathology is suspected.
Referral to a urologist for further evaluation and management, especially if the pain persists or worsens over time.